add custom plugin sample

Author: TwitchBronBron

brighterscript-with-custom-plugin/.vscode/settings.json

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+{
+    "brightscript.bsdk": "node_modules\\brighterscript"
+}

brighterscript-with-custom-plugin/bsc-custom-plugin.ts

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+import { CompilerPlugin, BscFile, isBrsFile } from 'brighterscript';
+
+// plugin factory
+export default function () {
+    return {
+        name: 'no-underscores',
+        // post-parsing validation
+        afterFileValidate: (file: BscFile) => {
+            if (isBrsFile(file)) {
+                // visit function statements and validate their name
+                file.parser.references.functionStatements.forEach((fun) => {
+                    if (fun.name.text.includes('_')) {
+                        file.addDiagnostics([{
+                            code: 'custom:9000',
+                            message: 'Do not use underscores in function names',
+                            range: fun.name.range,
+                            file
+                        }]);
+                    }
+                });
+            }
+        }
+    } as CompilerPlugin;
+};

brighterscript-with-custom-plugin/bsconfig.json

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+{
+    "rootDir": "src",
+    "plugins": [
+        "./bsc-custom-plugin.ts"
+    ],
+    "require": [
+        "ts-node/register"
+    ]
+}