[docgen-ts] Add getExportStatement()

[sainthkh]

• Part of Replace espree and doctrine with typescript in docgen. #21238

Description

Extract export statements from a JavaScript file.

How has this been tested?

Unit testing.

Screenshots

N/A

Types of changes

Breakdown of #21238.

Note

To incorporate type system, the AST of TypeScript is very different from other JS parsers.

To solve that problem, the simple getExportTokens() had to be re-written in this way.

gutenberg/packages/docgen/src/engine.js

Lines 23 to 30 in fe0d10c

	const getExportTokens = ( ast ) =>
	ast.body.filter( ( node ) =>
	[
	'ExportNamedDeclaration',
	'ExportDefaultDeclaration',
	'ExportAllDeclaration',
	].some( ( declaration ) => declaration === node.type )
	);

Checklist:

• My code is tested.
• My code follows the WordPress code style.
• [N/A] My code follows the accessibility standards.
• My code has proper inline documentation.
• [N/A] I've included developer documentation if appropriate.
• [N/A] I've updated all React Native files affected by any refactorings/renamings in this PR.

[sainthkh]

Close because we decided to use babel instead.