Build ECMAScript modules (#1479)

* Separate type import statement from module import statement

* Remove extension from import statements

* Build ECMAScript modules

* Add `cross-env` devDependency

* Use `babel-plugin-import-rename` instead of custom plugin

* Improve `.babelrc` style and add comments

* Update README.md

* Change file extension to `.mjs`

Comply with Node.js spec. Use extensionless package:main.

* Enforce only ESM compatible imports

* Dedupe packages

* Add `unicodeMake.js` to overrides:excludedFiles

* Fix .eslintrc merge conflict

* Use rollup to bundle ES module

* Remove `eslint-plugin-import`

* Change build directory to `dist`

* Change build directory to `dist`

* Change build directory

* Move docs from README.md to browser.md

* Update update-sri.js

* Revert update-sri.js

* Revert update-sri.js

* Update .eslintrc

* Remove SSH key testing

rollup.config.js

@@ -0,0 +1,14 @@
+import babel from 'rollup-plugin-babel';
+
+process.env.BABEL_ENV = 'esm';
+
+export default {
+    input: 'katex.js',
+    output: {
+        file: 'dist/katex.mjs',
+        format: 'es',
+    },
+    plugins: [
+        babel(),
+    ],
+};