fix: Import Expo FS conditionally to work with Expo 54 (#699)

## Description

Expo 54 introduces a new FileSystem API, deprecating the ones used in
our codebase. The old APIs can still be accessed under
`expo-file-system/legacy`. This is a temporary fix to work with old Expo
versions.

### Introduces a breaking change?

- [ ] Yes
- [x] No

### Type of change

- [x] Bug fix (change which fixes an issue)
- [ ] New feature (change which adds functionality)
- [ ] Documentation update (improves or adds clarity to existing
documentation)
- [ ] Other (chores, tests, code style improvements etc.)

### Tested on

- [x] iOS
- [x] Android

### Testing instructions

<!-- Provide step-by-step instructions on how to test your changes.
Include setup details if necessary. -->

### Screenshots

<!-- Add screenshots here, if applicable -->

### Related issues

<!-- Link related issues here using #issue-number -->

### Checklist

- [ ] I have performed a self-review of my code
- [ ] I have commented my code, particularly in hard-to-understand areas
- [ ] I have updated the documentation accordingly
- [ ] My changes generate no new warnings

### Additional notes

<!-- Include any additional information, assumptions, or context that
reviewers might need to understand this PR. -->

Author: chmjkb

packages/react-native-executorch/src/constants/directories.ts

@@ -1,3 +1,5 @@
-import { documentDirectory } from 'expo-file-system';
+import { importLegacyExpoFSModules } from '../utils/ResourceFetcher';
+
+const { documentDirectory } = importLegacyExpoFSModules();
 
 export const RNEDirectory = `${documentDirectory}react-native-executorch/`;

packages/react-native-executorch/src/controllers/LLMController.ts

@@ -1,9 +1,11 @@
 import { ResourceSource } from '../types/common';
-import { ResourceFetcher } from '../utils/ResourceFetcher';
+import {
+  importLegacyExpoFSModules,
+  ResourceFetcher,
+} from '../utils/ResourceFetcher';
 import { ETError, getError } from '../Error';
 import { Template } from '@huggingface/jinja';
 import { DEFAULT_CHAT_CONFIG } from '../constants/llmDefaults';
-import { readAsStringAsync } from 'expo-file-system';
 import {
   ChatConfig,
   GenerationConfig,
@@ -14,6 +16,7 @@ import {
 } from '../types/llm';
 import { parseToolCall } from '../utils/llm';
 import { Logger } from '../common/Logger';
+const { readAsStringAsync } = importLegacyExpoFSModules();
 
 export class LLMController {
   private nativeModule: any;

packages/react-native-executorch/src/utils/ResourceFetcher.ts

@@ -27,7 +27,27 @@
  *   - Implements linked list behavior via the `.next` attribute
  *   - Automatically processes subsequent downloads when `.next` contains a valid resource
  */
-import {
+import type * as FileSystemTypes from 'expo-file-system';
+
+export function importLegacyExpoFSModules() {
+  let FileSystem: typeof FileSystemTypes;
+
+  try {
+    const expoPkg = require('expo/package.json');
+    const sdkVersion = expoPkg.version.split('.')[0];
+
+    if (Number(sdkVersion) > 53) {
+      FileSystem = require('expo-file-system/legacy');
+    } else {
+      FileSystem = require('expo-file-system');
+    }
+  } catch (e) {
+    throw new Error('Expo must be installed to use react-native-executorch');
+  }
+  return FileSystem;
+}
+
+const {
   cacheDirectory,
   copyAsync,
   createDownloadResumable,
@@ -37,7 +57,8 @@ import {
   EncodingType,
   deleteAsync,
   readDirectoryAsync,
-} from 'expo-file-system';
+} = importLegacyExpoFSModules();
+
 import { Asset } from 'expo-asset';
 import { Platform } from 'react-native';
 import { RNEDirectory } from '../constants/directories';

packages/react-native-executorch/src/utils/ResourceFetcherUtils.ts

@@ -1,16 +1,14 @@
-/**
- * @internal
- */
-
-import {
-  DownloadResumable,
-  getInfoAsync,
-  makeDirectoryAsync,
-} from 'expo-file-system';
+import type * as FileSystemTypes from 'expo-file-system';
 import { RNEDirectory } from '../constants/directories';
 import { ResourceSource } from '../types/common';
 import { Asset } from 'expo-asset';
 import { Logger } from '../common/Logger';
+import { importLegacyExpoFSModules } from './ResourceFetcher';
+
+/**
+ * @internal
+ */
+const { getInfoAsync, makeDirectoryAsync } = importLegacyExpoFSModules();
 
 export const enum HTTP_CODE {
   OK = 200,
@@ -42,7 +40,7 @@ export interface ResourceSourceExtended {
 }
 
 export interface DownloadResource {
-  downloadResumable: DownloadResumable;
+  downloadResumable: FileSystemTypes.DownloadResumable;
   status: DownloadStatus;
   extendedInfo: ResourceSourceExtended;
 }
@@ -75,7 +73,7 @@ export namespace ResourceFetcherUtils {
     let totalLength = 0;
     let previousFilesTotalLength = 0;
     for (const source of sources) {
-      const type = await ResourceFetcherUtils.getType(source);
+      const type = ResourceFetcherUtils.getType(source);
       let length = 0;
       try {
         if (type === SourceType.REMOTE_FILE && typeof source === 'string') {