perf: reuse compiler process when using sass-embedded

This implements the Shared Resources proposal that was accepted and implemented in Dart Sass 1.70.0.
By reusing the same compiler process when compiling multiple files, this significantly improves
performance for tools like webpack.

Closes: #1163

src/index.js

@@ -74,7 +74,7 @@ async function loader(content) {
   let result;
 
   try {
-    result = await compile(sassOptions, options);
+    result = await compile(sassOptions);
   } catch (error) {
     // There are situations when the `file`/`span.url` property do not exist
     // Modern API

src/utils.js

@@ -650,6 +650,7 @@ function getWebpackImporter(loaderContext, implementation, includePaths) {
 }
 
 let nodeSassJobQueue = null;
+let sassEmbeddedCompiler = null;
 
 /**
  * Verifies that the implementation and version of Sass is supported by this loader.
@@ -665,10 +666,16 @@ function getCompileFn(implementation, options) {
 
   if (isNewSass) {
     if (options.api === "modern") {
-      return (sassOptions) => {
+      return async (sassOptions) => {
         const { data, ...rest } = sassOptions;
 
-        return implementation.compileStringAsync(data, rest);
+        if (!sassEmbeddedCompiler) {
+          // Create a long-running compiler process that can be reused
+          // for compiling individual files.
+          sassEmbeddedCompiler = await implementation.initAsyncCompiler();
+        }
+
+        return sassEmbeddedCompiler.compileStringAsync(data, rest);
       };
     }