Fixes update-readme script (#15679)

This PR does two things:

- Fixes #15626 and #15680 by making the script synchronous. In #15200 we missed the fact that to fill tokens within the same file we need to execute docgen synchronously, otherwise, the last token process will overwrite the first.

- Updates our espree dependency to 4.0.0. When error reporting was back, we uncovered that introducing short Fragment syntax in #15120 caused docgen to fail. The reason is that the espree version we used didn't support that. This fixes it by upgrading it to one that does. A couple of restrictions:

    - espree uses acorn-jsx to power JSX parsing.
    - acorn-jsx@4.1.0 added support for JSX fragment short syntax (patched in 4.1.1).
    - espree@4.0.0 added acorn-jsx@4.1.1. We should use this at a minimum.
    - espree@4.1.0 added acorn@6 and acorn-jsx@5 for parsing. This caused an error I couldn't identify the source.
    - espree@5.0.0 removed support for the attachComment. We use this for collocating the JSDoc comment with the proper export statement. Without this, we can't migrate to espree@5.0.0.

bin/update-readmes.js

@@ -1,8 +1,7 @@
 #!/usr/bin/env node
 
 const { join } = require( 'path' );
-const { promisify } = require( 'util' );
-const spawn = promisify( require( 'child_process' ).spawn );
+const spawnSync = require( 'child_process' ).spawnSync;
 
 const packages = [
 	'a11y',
@@ -39,15 +38,17 @@ const packages = [
 	'wordcount',
 ];
 
-Promise.all( packages.map( ( entry ) => {
+packages.forEach( ( entry ) => {
 	if ( ! Array.isArray( entry ) ) {
 		entry = [ entry, { 'Autogenerated API docs': 'src/index.js' } ];
 	}
 
 	const [ packageName, targets ] = entry;
 
-	return Promise.all( Object.entries( targets ).map( async ( [ token, path ] ) => {
-		const { status, stderr } = await spawn(
+	Object.entries( targets ).forEach( ( [ token, path ] ) => {
+		// Each target operates over the same file, so it needs to be processed synchronously,
+		// as to make sure the processes don't overwrite each other.
+		const { status, stderr } = spawnSync(
 			join( __dirname, '..', 'node_modules', '.bin', 'docgen' ),
 			[
 				join( 'packages', packageName, path ),
@@ -60,10 +61,8 @@ Promise.all( packages.map( ( entry ) => {
 		);
 
 		if ( status !== 0 ) {
-			throw stderr.toString();
+			process.stderr.write( `${ packageName } ${ stderr.toString() }\n` );
+			process.exit( 1 );
 		}
-	} ) );
-} ) ).catch( ( error ) => {
-	process.stderr.write( `${ error }\n` );
-	process.exit( 1 );
+	} );
 } );

package.json

@@ -92,7 +92,7 @@
 		"doctrine": "2.1.0",
 		"enzyme": "3.9.0",
 		"eslint-plugin-jest": "21.5.0",
-		"espree": "3.5.4",
+		"espree": "4.0.0",
 		"fbjs": "0.8.17",
 		"glob": "7.1.2",
 		"husky": "0.14.3",