Experiment with making ZLS build runner logic a build step

[SuperAuguste]

Solves many versioning issues as well as custom steps/generated files. Thanks @andrewrk for the suggestion. :)

Try it out with https://github.com/zigtools/zls-as-step-demo

• [x] Refresh build config when JSON manifest is changed
• [ ] Add test based on demo repo
• [ ] Add back pkg-config logic

~~Add extension integration (behavior: when possible, run zig build install-zls and use ZLS from zig-out)~~

[codecov[bot]]

Codecov Report

Attention: 4 lines in your changes are missing coverage. Please review.

Comparison is base (b328500) 75.60% compared to head (95e842a) 75.59%.

Files	Patch %	Lines
src/Server.zig	60.00%	4 Missing :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1650      +/-   ##
==========================================
- Coverage   75.60%   75.59%   -0.02%     
==========================================
  Files          33       33              
  Lines        9031     9039       +8     
==========================================
+ Hits         6828     6833       +5     
- Misses       2203     2206       +3     

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[SuperAuguste]

Sorry, undrafted prematurely; still some TODOs and testing stuff I want to address.