js.Build: Add SourceMap flag with inline option

Added a flag to allow turning on sourcemap in ESBuild. The current support
can only support inline or true as value for sourcemap. This is because
the way ESBuild is invoked it doesn't have a separate output path
to write the mapfile external to the asset pipeline. Add disable for "" and "0".
Add test script and make sure mage check passes.

Fixes #7607

resources/resource_transformers/js/build_test.go

@@ -63,4 +63,18 @@ func TestToBuildOptions(t *testing.T) {
 		Stdin:             &api.StdinOptions{},
 	})
 
+	opts, err = toBuildOptions(Options{
+		Target: "es2018", Format: "cjs", Minify: true, mediaType: media.JavascriptType,
+		SourceMap: "inline"})
+	c.Assert(err, qt.IsNil)
+	c.Assert(opts, qt.DeepEquals, api.BuildOptions{
+		Bundle:            true,
+		Target:            api.ES2018,
+		Format:            api.FormatCommonJS,
+		MinifyIdentifiers: true,
+		MinifySyntax:      true,
+		MinifyWhitespace:  true,
+		Sourcemap:         api.SourceMapInline,
+		Stdin:             &api.StdinOptions{},
+	})
 }