feat: allow setting getCustomTransformers as a path to a module (#531)

When using forked checker mode, it's impossible to pass a custom
function to the checker via the options as it's a different process. To
get around this, we can pass a path to the module that is then exceuted
once the checker is initialized.

src/checker/runtime.ts

@@ -164,10 +164,28 @@ function createChecker(receive: (cb: (msg: Req) => void) => void, send: (msg: Re
 
     class Host implements ts.LanguageServiceHost {
         filesRegex: RegExp;
-        getCustomTransformers = loaderConfig.getCustomTransformers;
+        getCustomTransformers?: () => ts.CustomTransformers | undefined;
 
         constructor(filesRegex: RegExp) {
             this.filesRegex = filesRegex;
+
+            let {getCustomTransformers} = loaderConfig;
+
+            if (typeof getCustomTransformers === "function") {
+                this.getCustomTransformers = getCustomTransformers;
+            } else if (typeof getCustomTransformers === "string") {
+                try {
+                    getCustomTransformers = require(getCustomTransformers);
+                } catch (err) {
+                    throw new Error(`Failed to load customTransformers from "${loaderConfig.getCustomTransformers}": ${err.message}`)
+                };
+
+                if (typeof getCustomTransformers !== "function") {
+                    throw new Error(`Custom transformers in "${loaderConfig.getCustomTransformers}" should export a function, got ${typeof getCustomTransformers}`)
+                };
+
+                this.getCustomTransformers = getCustomTransformers;
+            };
         }
 
         getProjectVersion() { return projectVersion.toString(); }
@@ -400,7 +418,8 @@ function createChecker(receive: (cb: (msg: Req) => void) => void, send: (msg: Re
         const trans = compiler.transpileModule(files.get(fileName).text, {
             compilerOptions: compilerOptions,
             fileName,
-            reportDiagnostics: false
+            reportDiagnostics: false,
+            transformers: host.getCustomTransformers ? host.getCustomTransformers() : undefined,
         });
 
         return {

src/interfaces.ts

@@ -28,7 +28,7 @@ export interface LoaderConfig {
     debug?: boolean;
     reportFiles?: string[];
     context?: string;
-    getCustomTransformers?(): ts.CustomTransformers | undefined;
+    getCustomTransformers?: string | (() => ts.CustomTransformers | undefined);
 }
 
 export interface OutputFile {