[DevTools] Updated source map extension format + more precise types (facebook#22073)

## Summary

Follow up from facebook#22010.

As suggested by @motiz88, update the way the react sources metadata is stored within the fb sources metadata. Specifically,  instead of `x_facebook_sources` directly containing a hook map in the second position of the metadata tuple for a given source, it contains the react sources metadata itself, which is also a tuple of react sources metadata for a given source, and which contains the hook map in the first position. This way the react sources metadata tuple can be extended to contain more react-specific metadata without taking up more positions in the top-level facebook sources metadata.

As part of this change:
- Adds more precise Flow types, mostly borrowed from Metro
- Fixes the facebook sources field name (we were using `x_fb_sources` but it should be `x_facebook_sources`

## Test Plan

- yarn flow
- yarn test
- yarn test-build-devtools

Author: Juan

packages/react-devtools-extensions/src/SourceMapTypes.js

@@ -0,0 +1,50 @@
+/*
+ * Copyright (c) Facebook, Inc. and its affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ *
+ * @flow strict-local
+ */
+
+import type {EncodedHookMap} from './generateHookMap';
+
+export type ReactSourceMetadata = [?EncodedHookMap];
+export type ReactSourcesArray = $ReadOnlyArray<?ReactSourceMetadata>;
+
+export type FBSourceMetadata = [?{...}, ?ReactSourceMetadata];
+export type FBSourcesArray = $ReadOnlyArray<?FBSourceMetadata>;
+
+export type BasicSourceMap = {|
+  +file?: string,
+  +mappings: string,
+  +names: Array<string>,
+  +sourceRoot?: string,
+  +sources: Array<string>,
+  +sourcesContent?: Array<?string>,
+  +version: number,
+  +x_facebook_sources?: FBSourcesArray,
+  +x_react_sources?: ReactSourcesArray,
+|};
+
+export type IndexMapSection = {
+  map: IndexMap | BasicSourceMap,
+  offset: {
+    line: number,
+    column: number,
+    ...
+  },
+  ...
+};
+
+export type IndexMap = {|
+  +file?: string,
+  +mappings?: void, // avoids SourceMap being a disjoint union
+  +sourcesContent?: void,
+  +sections: Array<IndexMapSection>,
+  +version: number,
+  +x_facebook_sources?: FBSourcesArray,
+  +x_react_sources?: ReactSourcesArray,
+|};
+
+export type MixedSourceMap = IndexMap | BasicSourceMap;