Make tree alpha APIs actually usable (#22483)

## Description

`@fluidframework/tree` and `fluid-framework` now have a `/alpha` import
path where their `@alpha` APIs are exported.

.changeset/light-cows-think.md

@@ -0,0 +1,10 @@
+---
+"fluid-framework": minor
+"@fluidframework/tree": minor
+---
+---
+"section": "tree"
+---
+Add /alpha import path to @fluidframework/tree and fluid-framework packages
+
+`@fluidframework/tree` and `fluid-framework` now have a `/alpha` import path where their `@alpha` APIs are exported.

common/build/build-common/api-extractor-report.esm.current.json

@@ -5,8 +5,8 @@
 	// or alpha.d.ts) rollup file.
 	"mainEntryPointFilePath": "set-mainEntryPointFilePath-directly-in-leaf-config-file",
 	"apiReport": {
-		// NOTE: "alpha" is temporarily not included to reduce noise while there are no uses.
-		//  Once any package with /legacy also uses /alpha, "alpha" should be added in.
+		// NOTE: "alpha" is not included to reduce noise since there are few users.
+		// Packages wanting alpha API reports can override this to opt in.
 		"reportVariants": ["public", "beta"]
 	},
 	"docModel": {

packages/dds/tree/README.md

@@ -39,6 +39,8 @@ To access the `public` ([SemVer](https://semver.org/)) APIs, import via `@fluidf
 
 To access the `beta` APIs, import via `@fluidframework/tree/beta`.
 
+To access the `alpha` APIs, import via `@fluidframework/tree/alpha`.
+
 To access the `legacy` APIs, import via `@fluidframework/tree/legacy`.
 
 ## API Documentation

packages/dds/tree/api-extractor/api-extractor-lint-alpha.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/alpha.d.ts"
+}

packages/dds/tree/api-extractor/api-extractor-lint-alpha.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/alpha.d.ts"
+}

packages/dds/tree/api-extractor/api-extractor.current.json

@@ -1,5 +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.current.json",
-	"mainEntryPointFilePath": "<projectFolder>/lib/beta.d.ts"
+	"apiReport": {
+		"reportVariants": ["public", "beta", "alpha"]
+	},
+	"mainEntryPointFilePath": "<projectFolder>/lib/alpha.d.ts"
 }