Re-use TypeScriptLanguageServiceHostfor monaco and treeshaker build steps

For #270408

There are currently duplicated `LanguageServiceHost` definitions in our build folder. As far as I can tell they are the same except for the default lib name

As part of #270408 I'll have to touch the service hosts and having a single definition will make this easier

build/lib/typeScriptLanguageServiceHost.ts

@@ -0,0 +1,67 @@
+/*---------------------------------------------------------------------------------------------
+ *  Copyright (c) Microsoft Corporation. All rights reserved.
+ *  Licensed under the MIT License. See License.txt in the project root for license information.
+ *--------------------------------------------------------------------------------------------*/
+
+import ts from 'typescript';
+
+export type ILibMap = Map</*libName*/ string, string>;
+export type IFileMap = Map</*fileName*/ string, string>;
+
+/**
+ * A TypeScript language service host
+ */
+export class TypeScriptLanguageServiceHost implements ts.LanguageServiceHost {
+
+	constructor(
+		private readonly ts: typeof import('typescript'),
+		private readonly libs: ILibMap,
+		private readonly files: IFileMap,
+		private readonly compilerOptions: ts.CompilerOptions,
+		private readonly defaultLibName: string,
+	) { }
+
+	// --- language service host ---------------
+	getCompilationSettings(): ts.CompilerOptions {
+		return this.compilerOptions;
+	}
+	getScriptFileNames(): string[] {
+		return [
+			...this.libs.keys(),
+			...this.files.keys(),
+		];
+	}
+	getScriptVersion(_fileName: string): string {
+		return '1';
+	}
+	getProjectVersion(): string {
+		return '1';
+	}
+	getScriptSnapshot(fileName: string): ts.IScriptSnapshot {
+		if (this.files.has(fileName)) {
+			return this.ts.ScriptSnapshot.fromString(this.files.get(fileName)!);
+		} else if (this.libs.has(fileName)) {
+			return this.ts.ScriptSnapshot.fromString(this.libs.get(fileName)!);
+		} else {
+			return this.ts.ScriptSnapshot.fromString('');
+		}
+	}
+	getScriptKind(_fileName: string): ts.ScriptKind {
+		return this.ts.ScriptKind.TS;
+	}
+	getCurrentDirectory(): string {
+		return '';
+	}
+	getDefaultLibFileName(_options: ts.CompilerOptions): string {
+		return this.defaultLibName;
+	}
+	isDefaultLibFileName(fileName: string): boolean {
+		return fileName === this.getDefaultLibFileName(this.compilerOptions);
+	}
+	readFile(path: string, _encoding?: string): string | undefined {
+		return this.files.get(path) || this.libs.get(path);
+	}
+	fileExists(path: string): boolean {
+		return this.files.has(path) || this.libs.has(path);
+	}
+}