feat: support json and css import assertions

Note: this is known to fail some tests

due to a bug in the acorn-import-assertions dependency.

We are planning to contribute upstream to correct it.

Author: swiing

.gitignore

@@ -1,4 +1,5 @@
 node_modules/
+dist/
 
 .DS_Store
 .eslintcache

README.md

@@ -3,3 +3,111 @@
 # rollup-plugin-import-assertions
 
 🍣 A Rollup plugin which bundles [import assertions](https://github.com/tc39/proposal-import-assertions).
+
+Two types of assertions are supported: `json` and `css`.
+
+Currently, dynamic imports are not supported (PR welcomed).
+
+## Usage
+
+Create a `rollup.config.js` [configuration file](https://www.rollupjs.org/guide/en/#configuration-files) and import the plugin:
+
+```js
+import importAssertions from 'rollup-plugin-import-assertions';
+
+export default {
+  input: 'src/index.js',
+  output: {
+    dir: 'output',
+    format: 'cjs'
+  },
+  plugins: [importAssertions()]
+};
+```
+
+Then call `rollup` either via the [CLI](https://www.rollupjs.org/guide/en/#command-line-reference) or the [API](https://www.rollupjs.org/guide/en/#javascript-api).
+
+With an accompanying file `src/index.js`, the local `package.json` file would now be importable as seen below:
+
+```js
+// src/index.js
+import pkg from './package.json' assert { type: 'json' };
+console.log(`running version ${pkg.version}`);
+```
+
+It is also possible to import css stylesheets, typically when designing web components:
+
+```js
+// src/mycomponent.js
+import style from './style.css' assert { type: 'css' };
+
+class MyElement extends HTMLElement {
+  constructor() {
+    super();
+    const root = this.attachShadow({ mode: 'open' });
+    root.adoptedStyleSheets = [ styles ];
+    root.innerHTML = `<div>My custom element</div>`;
+  }
+}
+
+customElements.define('my-element', MyElement);
+```
+
+## Options
+
+For the `json` type of assertions, this plugin accepts the same options
+as those of [@rollup/plugin-json](https://github.com/rollup/plugins/tree/master/packages/json/).
+This makes it straight-forward to move to import assertions, should one wish so.
+
+For the `css` type of assertions, this plugin accepts the usual `include` and `exclude` options.
+
+### `compact` (type: 'json')
+
+Type: `Boolean`<br>
+Default: `false`
+
+If `true`, instructs the plugin to ignore `indent` and generates the smallest code.
+
+### `exclude`
+
+Type: `String` | `Array[...String]`<br>
+Default: `null`
+
+A [minimatch pattern](https://github.com/isaacs/minimatch), or array of patterns, which specifies the files in the build the plugin should _ignore_. By default no files are ignored.
+
+### `include`
+
+Type: `String` | `Array[...String]`<br>
+Default: `null`
+
+A [minimatch pattern](https://github.com/isaacs/minimatch), or array of patterns, which specifies the files in the build the plugin should operate on. By default all files are targeted.
+
+### `indent` (type: 'json')
+
+Type: `String`<br>
+Default: `'\t'`
+
+Specifies the indentation for the generated default export.
+
+### `namedExports` (type: 'json')
+
+Type: `Boolean`<br>
+Default: `true`
+
+If `true`, instructs the plugin to generate a named export for every property of the JSON object.
+
+### `preferConst` (type: 'json')
+
+Type: `Boolean`<br>
+Default: `false`
+
+If `true`, instructs the plugin to declare properties as variables, using either `var` or `const`. This pertains to tree-shaking.
+
+## Credits
+
+Credits to:
+
+- [@rollup/plugin-json](https://github.com/rollup/plugins/tree/master/packages/json/),
+on top of which this plugin shamelessly builds.
+
+- [rollup-plugin-import-assert](https://github.com/calebdwilliams/rollup-plugin-import-assert) which was inspirational to start with.