Set missing treesitter highlight groups

[cherryman]

Apologies for the churn that treesitter has caused this project :sweat_smile:.

Commit 07e70d5 is missing some highlight groups needed for some languages like Rust. I've added the missing groups, while trying to be authentic to the theme. Since treesitter is much more granular, it's likely that there's some minor divergence from non-treesitter colours.

Some notable changes:

• Changed @constant.builtin from cyan to orange. This includes keywords like None in Python or undefined in JavaScript, which are semantically closer to items like constants, numbers, and booleans, which are orange. I can change this back if desired.
• Changed @boolean from pink to orange. Same rationale as above.
• Changed @variable.member from white to fg. I suspect this was likely a mistake.
• Added @markup.* colours, trying to match those from the existing markdown colours.

cc @cpmsmith