feat: treat iota as predeclared identifier

[kawaemon]

I'm using this parser with nvim-treesitter, and I noticed that iota is not highlighted unlike true or other constants. This PR defines iota as predeclared identifier on grammer.js and on highlights query as well.

Checklist:

• [x] All tests pass in CI.
• [x] There are sufficient tests for the new fix/feature.
• [x] Grammar rules have not been renamed unless absolutely necessary.
• [x] The conflicts section hasn't grown too much.
• [x] The parser size hasn't grown too much (check the value of STATE_COUNT in src/parser.c). STATE_COUNT didn't change.

[maxbrunsfeld]

Thanks for this @kawaemon, and thanks for merging PRs @aryx 🙏 .