Unify asset building and use find_program to find NPM

[chewi]

Description

Firstly, this unifies the asset building by leveraging CMake's env command. add_custom_target can also accept multiple commands to be run in sequence. It isn't clear how quoting applies here, but I've tested it, and it seems to be behave as expected when spaces are present.

Secondly, this uses find_program to find NPM so it can be overridden. This is useful for Gentoo, which needs to be able to do entirely offline builds, because it can override this with /bin/true while shipping pre-compiled assets. Gentoo has tried to ship cached NPM modules instead, but it turns out these are very sensitive to the NPM version. I also tried swapping in Yarn, which handles caching better, but ran into other issues.

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Dependency update (updates to dependencies)
• Documentation update (changes to documentation)
• Repository update (changes to repository files, e.g. .github/...)

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have added or updated the in code docstring/documentation-blocks for new or existing methods/components

Branch Updates

LizardByte requires that branches be up-to-date before merging. This means that after any PR is merged, this branch
must be updated before it can be merged. You must also
Allow edits from maintainers.

• I want maintainers to keep my branch updated

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 7.00%. Comparing base (4e49db9) to head (4c44738).

Additional details and impacted files

@@            Coverage Diff             @@
##           nightly   #2522      +/-   ##
==========================================
- Coverage     7.02%   7.00%   -0.03%     
==========================================
  Files           87      87              
  Lines        17675   17669       -6     
  Branches      8395    8395              
==========================================
- Hits          1242    1238       -4     
- Misses       13709   13710       +1     
+ Partials      2724    2721       -3     

Flag	Coverage Δ
Linux	5.42% <ø> (+0.05%)	⬆️
Windows	2.60% <ø> (ø)
macOS-12	?
macOS-13	?
macOS-14	8.28% <ø> (ø)

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see 5 files with indirect coverage changes