frint: Expose class methods on the app instance level

[rbardini]

What's done

• Add optional methods (Object) property to AppOptions, which has its properties added to the app's this object.
• Add app.getMethods method which returns the app's methods object.

Why

Allow access to public methods directly from the app instance, besides the options property.

Closes #426.

[codecov[bot]]

Codecov Report

Merging #431 into master will increase coverage by <.01%. The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #431      +/-   ##
==========================================
+ Coverage   97.75%   97.76%   +<.01%     
==========================================
  Files         112      112              
  Lines        4366     4380      +14     
==========================================
+ Hits         4268     4282      +14     
  Misses         98       98

Impacted Files	Coverage Δ
packages/frint/src/App.spec.ts	100% <100%> (ø)	:arrow_up:
packages/frint/src/App.ts	99.37% <100%> (+0.02%)	:arrow_up:

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[fahad19]

Great stuff, @rbardini! 💖