Split up the execution code into multiple files

[arduano]

Depends on #109

Split up the execution code to be in multiple files, rather than concerning 1 file with like 5 different tasks.

This should make future changes/contributions a lot easier to make and review.

Also, I disabled -Dwarnings in the flake. I assume the intention was to have warnings throw errors in CI, but that's better done with --deny "warnings" instead.

[arduano]

FYI the next plans I have:

• Refactor the way execution dispatch works, making it much easier to have a shared execution environment that a module could be executed in directly to get the nixjs expression result
• Use the above to make an import builtin, which evaluates a file/expression into an expression result and returns it
• Do some cleanups around other parts, e.g. emitting js could be done much cleaner using write! expressions
• Add some helper abstractions around v8 interaction

[urbas]

Oh, I see what happened. This pull request was targeted to the inline-nixjs-rt branch rather than the master branch. When I merged the inline-nixjs-rt branch into master I also deleted the inline-nixjs-rt branch (that's what I always do, to not accumulate branches). The act of deleting the inline-nixjs-rt branch closed this PR (rather than preventing the deletion of the branch).

[arduano]

Yeah there should be an option to re-link the PR to master when you merge the other branch into master iirc