Use cargo-xtask for automating builds

[lrlna]

Description

Our current build.rs is handling a lot of various functionality pertaining to the release, installers and npm. To automate and test these separate pieces, we should consider switching to using [cargo-xtask](https://github.com/matklad/cargo-xtask).

I see us have a few xtasks:

• cargo xtask npm - to generate all applicable work for our npm package
• cargo xtask release -- to handle release management and prep
• cargo xtask install -- to handle installations
• maybe a few more??

Prior Art

A few good examples for us to look at:

• cargo xtask itself has a good hello-world example.
• rust-analyzer utilises it a bunch for codegen, release and installation management. Tools of interest:
  • tidy
  • release
  • install

[EverlastingBugstopper]

cargo xtask dist seems to be the pattern for making release stuff happen, and cargo xtask ci could include running our clippy + tests stuff, everything currently in lint.yml could be slurped up here.