chore(core): Refactor options package names

[irvinlim]

Updates naming of symbols and files in the core/options package to be more standardized.

Also exposed a GetOptionDefaultValue which returns the raw default value prior to evaluation.

[codecov]

Codecov Report

Merging #66 (c5ebcd7) into main (a55d7a2) will decrease coverage by 0.13%.
The diff coverage is 87.64%.

@@            Coverage Diff             @@
##             main      #66      +/-   ##
==========================================
- Coverage   61.06%   60.92%   -0.14%     
==========================================
  Files         168      170       +2     
  Lines        8447     8486      +39     
==========================================
+ Hits         5158     5170      +12     
- Misses       3047     3070      +23     
- Partials      242      246       +4     

Impacted Files	Coverage Δ
pkg/core/options/substitution.go	100.00% <ø> (ø)
pkg/core/options/default.go	65.16% <63.15%> (-11.76%)	⬇️
pkg/core/options/mutation.go	95.00% <95.00%> (ø)
pkg/core/options/validation.go	97.93% <97.93%> (-2.07%)	⬇️
pkg/core/options/options.go	90.24% <100.00%> (-4.02%)	⬇️
pkg/core/options/util.go	100.00% <100.00%> (ø)
pkg/execution/mutation/mutation.go	88.02% <100.00%> (ø)

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