Remove the fs dependency in the TS build.

[charlespwd]

This should allow users who import dist/parser/stage-2-ast to not make their compiler freak out.

Before:

• Have a grammar/liquid-html.ohm.js that reads liquid-html.ohm with fs
• Have a build/shims/liquid-html.ohm file that replaces the fs.readFile with the contents of the file
• Have a webpack alias that changes ../../grammar/liquid-html.ohm.js with the shim

After:

• Have a build script that pastes the content of grammar/liquid-html.ohm into grammar/liquid-html.ohm.js as a module.exports = String.raw``;
• Run that script before building
• Run that script before running tests

The advantage now is that there's no fs dependency in the TS build.