WIP: emscripten toolchain

[GPMueller]

• [x] add Toolchain base class
• [x] add Emscripten toolchain test
• [x] update CI to provide emscripten
• [x] add documentation page
• [ ] refactor the way default flags etc. work to make custom toolchains safer and easier to create

Closes #123.

[codecov[bot]]

Codecov Report

Merging #129 (c13031c) into master (2c671f5) will decrease coverage by 1.30%. The diff coverage is 91.56%.

@@            Coverage Diff             @@
##           master     #129      +/-   ##
==========================================
- Coverage   79.56%   78.26%   -1.31%     
==========================================
  Files          17       17              
  Lines        1199     1256      +57     
==========================================
+ Hits          954      983      +29     
- Misses        245      273      +28     

Impacted Files	Coverage Δ
clang_build/environment.py	87.71% <86.66%> (-2.61%)	:arrow_down:
clang_build/toolchain.py	80.00% <92.30%> (-2.66%)	:arrow_down:
clang_build/cli.py	53.60% <100.00%> (+1.99%)	:arrow_up:
clang_build/target.py	80.51% <100.00%> (ø)
clang_build/single_source.py	42.42% <0.00%> (-19.70%)	:arrow_down:
clang_build/progress_bar.py	73.33% <0.00%> (-6.67%)	:arrow_down:

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[GPMueller]

Created a follow-up issue, #137, for the remaining TODO. As the feature already basically works, I will now merge this PR.