Enable symlinks (resolver.unstable_enableSymlinks) by default

Summary:
Flip the default for `unstable_enableSymlinks` to true. The option was made available in React Native 0.72 and it's been trialled by the community to the extent that we can have good confidence in it (though there are still devx gaps to guide users towards a suitable configuration).

This is also an effective prerequisite for disabling Haste/global package resolution by default in OSS, which currently allows workspaces to mostly-work in the absence of symlink support.

Closes facebook#1

Changelog:
```
 - **[Feature]**: Enable resolution through symlinks (previously experimental `unstable_enableSymlinks`)
```

Differential Revision: D48777855

fbshipit-source-id: 8da1ab84d2971736ec2560aae4a82294aec6da9f

docs/Configuration.md

@@ -405,11 +405,11 @@ When enabled, Metro traverses symlinks during module and asset [resolution](./Re
 
 For example, if you have a Metro project within a [Yarn workspace](https://classic.yarnpkg.com/lang/en/docs/workspaces/) (a subdirectory of a Yarn workspace root), it's likely you'll want to include your workspace *root* path in your configured [`watchFolders`](#watchfolders) so that Metro can resolve other workspaces or hoisted `node_modules`. Similarly, to use linked packages, you'll need to list those package source locations (or a containing directory) in [`watchFolders`](#watchfolders).
 
-Defaults to `false`.
+Defaults to `true` for Metro versions *after* v0.78.1.
 
 :::note
 
-In a future release of Metro, this option will become `true` by default.
+In a future release of Metro, this option will be removed (symlink support will be always-on).
 
 :::
 

packages/metro-config/src/__tests__/__snapshots__/loadConfig-test.js.snap

@@ -84,7 +84,7 @@ Object {
       ],
     },
     "unstable_enablePackageExports": false,
-    "unstable_enableSymlinks": false,
+    "unstable_enableSymlinks": true,
     "useWatchman": true,
   },
   "serializer": Object {
@@ -262,7 +262,7 @@ Object {
       ],
     },
     "unstable_enablePackageExports": false,
-    "unstable_enableSymlinks": false,
+    "unstable_enableSymlinks": true,
     "useWatchman": true,
   },
   "serializer": Object {
@@ -440,7 +440,7 @@ Object {
       ],
     },
     "unstable_enablePackageExports": false,
-    "unstable_enableSymlinks": false,
+    "unstable_enableSymlinks": true,
     "useWatchman": true,
   },
   "serializer": Object {
@@ -618,7 +618,7 @@ Object {
       ],
     },
     "unstable_enablePackageExports": false,
-    "unstable_enableSymlinks": false,
+    "unstable_enableSymlinks": true,
     "useWatchman": true,
   },
   "serializer": Object {

packages/metro-config/src/defaults/index.js

@@ -40,7 +40,7 @@ const getDefaultValues = (projectRoot: ?string): ConfigT => ({
     blockList: exclusionList(),
     dependencyExtractor: undefined,
     disableHierarchicalLookup: false,
-    unstable_enableSymlinks: false,
+    unstable_enableSymlinks: true,
     emptyModulePath: require.resolve(
       'metro-runtime/src/modules/empty-module.js',
     ),