Helper scripts to build and test CodeMirror packages.

The exports (build and watch) from this package build packages
that conform to the conventions of the various @codemirror packages.
They do the following:

• Mangle the code to convert our /// doc comments to /** */
  comments, so that TypeScript will not strip them.

• Run the TypeScript compiler, catching the output in memory.

• Run rollup and rollup-plugin-dts on the result to emit the CommonJS
  and ES modules, as well as a bundled .d.ts file, to dist/.

There's also a cm-buildhelper binary which builds the main file
specified as its first argument. This is used by the individual
packages in their prepare scripts.

NOTE: This tool will add a /*@__PURE__*/ annotation in front of
every top-level function call, to allow tree-shaking of things like
Facet.define(...). This is likely to break some styles of code
(which, say, uses a side-effecting helper function to build up some
top-level data structure), but I wasn't able to find another approach
for working with the crude current state of JS dead-code detection
tools.

The cm-runtests binary helps run tests. Given a list of directories,
it'll run ./test/test-*.js as plain mocha tests, and
./test/webtest-*.js using a Selenium headless browser.

You can also pass it a --server argument to start a test server that
you can interact with in your browser.