Flow: add decodedMap to Babel type definitions (#57302)

Summary:
Pull Request resolved: #57302

Adds missing types for `babel/generator`'s `result.decodedMap`, added to Babel in babel/babel#14497 (v7.17.10)

https://babeljs.io/docs/babel-generator/#result

We'll use this in Metro for more efficient source map generation.

Changelog: [Internal]

Reviewed By: GijsWeterings

Differential Revision: D109247419

fbshipit-source-id: 533a4093f9eb768970e98b6597fc9d842c66d027

Author: robhogan

flow-typed/npm/babel_v7.x.x.js

@@ -28,6 +28,24 @@ type _BabelSourceMapSegment = {
   ...
 };
 
+// A "decoded" source map (as produced by `@jridgewell/gen-mapping`), grouped by
+// generated line. Segment fields are all 0-based: generated column, source
+// index, source line, source column, name index.
+type _BabelDecodedSourceMapSegment =
+  | [number]
+  | [number, number, number, number]
+  | [number, number, number, number, number];
+
+type _BabelDecodedSourceMap = Readonly<{
+  file?: string,
+  mappings: Array<Array<_BabelDecodedSourceMapSegment>>,
+  names: Array<string>,
+  sourceRoot?: string,
+  sources: Array<string>,
+  sourcesContent?: Array<?string>,
+  version: number,
+}>;
+
 export type BabelSourceLocation = Readonly<{
   start: Readonly<{line: number, column: number}>,
   end: Readonly<{line: number, column: number}>,
@@ -1111,6 +1129,7 @@ declare module '@babel/generator' {
   declare export type GeneratorResult = {
     code: string,
     map: ?_BabelSourceMap,
+    decodedMap: ?_BabelDecodedSourceMap,
     rawMappings: ?Array<BabelSourceMapSegment>,
   };