Pretokenise JS functions included with `generate_js`

[gfwilliams]

Right now, in scripts/build_jswrapper.py we just include the JS files as-is

However we could pretokenise them and make them run faster as well as saving some space.

Only issue is our tools for pretokenising are either in JS or C, not Python (and I don't think we require Node.js to be installed for a compile currently), so I guess maybe we'd have to either:

• Have a tool written in C that uses jslNewTokenisedStringFromLexer then build and run that from scripts/build_jswrapper.py
• Pre-compile all the JS files we use (a bit like we pre-minify currently)