extensions/markdown-language-features/server/src/workspace.ts/VsCodeDocument/positionAt
class VsCodeDocument:	positionAt(offset: number): Position {
		if (this.inMemoryDoc) {
			return this.inMemoryDoc.positionAt(offset);
		}

		if (this.onDiskDoc) {
			return this.onDiskDoc.positionAt(offset);
		}

		throw new Error('Document has been closed');
	}