[CX-1180] feat: embed helpers

[hermansje]

Currently, using one xwhat library in another one requires quite some glue code using internals of the SCT system.

embed_xwhat or get_embed_chain_constructors would be the way to use this , we should probably only expose one way to do this.

This PR adds some helpers that makes it easier and more future proof to do this.

TODO

• Decide on exposed helper.
• Add tests (I first want to get feedback on the feature).
• Add a usage example.

[codecov]

Codecov Report

Merging #42 into master will increase coverage by 0.77%.
The diff coverage is 97.91%.

@@            Coverage Diff             @@
##           master      #42      +/-   ##
==========================================
+ Coverage   80.64%   81.42%   +0.77%     
==========================================
  Files          16       16              
  Lines         811      845      +34     
==========================================
+ Hits          654      688      +34     
  Misses        157      157

Impacted Files	Coverage Δ
protowhat/utils.py	100% <100%> (ø)	⬆️
protowhat/sct_syntax.py	95.76% <100%> (+1.31%)	⬆️
protowhat/State.py	85.18% <80%> (-0.27%)	⬇️

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[TimSangster]

Looks good!