Fix plaintext grammar parsing error

Refs: http://microsoft/vscode-textmate#214

src/services/resolver.ts

@@ -7,7 +7,7 @@
 import * as vscode from 'vscode';
 import * as vscodeTextmate from 'vscode-textmate';
 import { readFileText, loadMessageBundle } from '../util/loader';
-import { ContributorData } from '../util/contributes';
+import { ContributorData, plaintextGrammarDefinition } from '../util/contributes';
 import type { GrammarLanguageDefinition, LanguageDefinition } from '../util/contributes';
 
 const localize = loadMessageBundle();
@@ -27,7 +27,9 @@ export class ResolverService implements vscodeTextmate.RegistryOptions {
 	public async loadGrammar(scopeName: string): Promise<vscodeTextmate.IRawGrammar | null> {
 		if (scopeName === 'text') {
 			const text = JSON.stringify(plainTextGrammar);
-			return vscodeTextmate.parseRawGrammar(text);
+			const appRoot = vscode.Uri.file(vscode.env.appRoot);
+			const jsonPath = vscode.Uri.joinPath(appRoot, plaintextGrammarDefinition.path).path
+			return vscodeTextmate.parseRawGrammar(text, jsonPath);
 		}
 
 		const mapping = this._contributes.sources;

src/util/contributes.ts

@@ -69,22 +69,22 @@ export type ExtensionManifestContributionKey = 'textmate-languageservice-contrib
 
 export type ExtensionData = Record<string, vscode.Extension<unknown> | undefined>;
 
-const plaintextLanguage: LanguageDefinition = {
+export const plaintextLanguageDefinition: LanguageDefinition = {
 	id: 'plaintext',
 	extensions: ['.txt'],
 	aliases: [localize('plainText.alias', 'Plain Text'), 'text'],
 	mimetypes: ['text/plain']
 };
 
-const plaintextGrammar = {
+export const plaintextGrammarDefinition = {
 	language: 'plaintext',
-	path: 'vscode://blank',
+	path: './out/vs/editor/common/languages/plaintext.tmLanguage.json',
 	scopeName: 'text'
 };
 
 function getAllContributes() {
-	const languages: LanguageData = [plaintextLanguage];
-	const grammars: GrammarData = [plaintextGrammar];
+	const languages: LanguageData = [plaintextLanguageDefinition];
+	const grammars: GrammarData = [plaintextGrammarDefinition];
 	const sources = {
 		grammars: {} as ExtensionData,
 		languages: {} as ExtensionData