Export StagingMode as legacy/alpha (#25377)

Adds a legacy/alpha entrypoint to runtime-definitions, and exports
staging mode as legacy/alpha.

---------

Co-authored-by: Joshua Smithrud <54606601+Josmithr@users.noreply.github.com>

Author: anthony-murphy

.github/CODEOWNERS

@@ -58,7 +58,6 @@
 # API report changes
 /**/api-report/*.public.api.md                 @microsoft/fluid-cr-api
 /**/api-report/*.beta.api.md                   @microsoft/fluid-cr-api
-/**/api-report/*.legacy.*.api.md               @microsoft/fluid-cr-api
 /tools/**/api-report/*.api.md                  # Do not require API review for tools packages
 
 # Changesets and release notes

packages/runtime/runtime-definitions/api-extractor/api-extractor-lint-legacyAlpha.cjs.json

@@ -0,0 +1,5 @@
+{
+	"$schema": "https://developer.microsoft.com/json-schemas/api-extractor/v7/api-extractor.schema.json",
+	"extends": "<projectFolder>/../../../common/build/build-common/api-extractor-lint.entrypoint.json",
+	"mainEntryPointFilePath": "<projectFolder>/dist/legacyAlpha.d.ts"
+}

packages/runtime/runtime-definitions/api-extractor/api-extractor-lint-legacyAlpha.esm.json

@@ -0,0 +1,5 @@
+{
+	"$schema": "https://developer.microsoft.com/json-schemas/api-extractor/v7/api-extractor.schema.json",
+	"extends": "<projectFolder>/../../../common/build/build-common/api-extractor-lint.entrypoint.json",
+	"mainEntryPointFilePath": "<projectFolder>/lib/legacyAlpha.d.ts"
+}

packages/runtime/runtime-definitions/api-extractor/api-extractor.legacy.json

@@ -1,4 +1,8 @@
 {
 	"$schema": "https://developer.microsoft.com/json-schemas/api-extractor/v7/api-extractor.schema.json",
-	"extends": "<projectFolder>/../../../common/build/build-common/api-extractor-report.esm.legacy.json"
+	"extends": "<projectFolder>/../../../common/build/build-common/api-extractor-report.esm.legacy.json",
+	"mainEntryPointFilePath": "<projectFolder>/lib/legacyAlpha.d.ts",
+	"apiReport": {
+		"reportVariants": ["public", "beta", "alpha"]
+	}
 }