feat(plugin): load `default` transformer if loading named transformer without name

docs/dev/writing-plugins.md

@@ -223,6 +223,8 @@ Users may then extend the preset using `plugin:name`, e.g.:
 }
 ```
 
+If you expose a transformer named `default` it will be loaded when using the plugin as name, i.e. `my-plugin` and `my-plugin:default` is equivalent.
+
 ## Extend metadata
 
 Plugins can extend the available [element metadata](/usage/elements.html) by

src/config/config.spec.ts

@@ -400,6 +400,7 @@ describe("config", () => {
 					"mock-plugin-named",
 					() => ({
 						transformer: {
+							default: transform as Transformer,
 							foobar: transform as Transformer,
 						},
 					}),
@@ -409,6 +410,7 @@ describe("config", () => {
 					plugins: ["mock-plugin-unnamed", "mock-plugin-named"],
 					transform: {
 						"\\.unnamed$": "mock-plugin-unnamed",
+						"\\.default$": "mock-plugin-named",
 						"\\.named$": "mock-plugin-named:foobar",
 						"\\.nonplugin$": "mock-transform",
 					},
@@ -454,6 +456,25 @@ describe("config", () => {
 				`);
 			});
 
+			it("unnamed default", () => {
+				expect.assertions(1);
+				source.filename = "bar.default";
+				expect(config.transformSource(source)).toMatchInlineSnapshot(`
+					Array [
+					  Object {
+					    "column": 3,
+					    "data": "transformed from bar.default",
+					    "filename": "bar.default",
+					    "line": 2,
+					    "offset": 4,
+					    "transformedBy": Array [
+					      "mock-plugin-named",
+					    ],
+					  },
+					]
+				`);
+			});
+
 			it("non-plugin (regression test issue 54)", () => {
 				expect.assertions(1);
 				source.filename = "bar.nonplugin";

src/config/config.ts

@@ -601,6 +601,9 @@ export class Config {
 		}
 
 		if (typeof plugin.transformer !== "function") {
+			if (plugin.transformer.default) {
+				return plugin.transformer.default;
+			}
 			throw new ConfigError(
 				`Transformer "${name}" refers to unnamed transformer but plugin exposes only named.`
 			);