Feature/js/parallelize matrix build

[julianneswinoga]

Linking is a single-threaded task it does not effectively utilize multicore processors. By changing matrix_build.py to instead create N projects which each will build a solution in parallel we can avoid the single-core linking bottleneck. This scales very well with the number of processors. Building mksgenlv21:

Host	old time	new time
Personal server (N=40)	30m23s	6m29s
Laptop (N=8)	14m10s	6m54s

Our current GitHub Actions workflow only runs on dual core machines (N=2), so the speedup is marginal (but tangible) but this prepares us well for if we want to run the build process on a more powerful CI.

env	old time	new time
mksgenlv21	14m47s	9m55s
mksgenlv2	18m10s	10m9s
mksgenlv1	4m30s	3m13s
esp32	3m32s	3m3s
ramps	15m26s	9m55s