feat: implement dsl for creating pact matchers

[matthewshirley]

This PR implements a DSL for creating matchers based on the Pact Specification v3.0.0. Implemented methods:

• EqualTo
• SomethingLike
• IntegerLike
• DecimalLike
• Includes
• Null
• Term
• EachLike

[codecov-commenter]

Codecov Report

Merging #2 (86acb65) into main (f241fe3) will increase coverage by 1.90%.
The diff coverage is 93.54%.

@@            Coverage Diff             @@
##             main       #2      +/-   ##
==========================================
+ Coverage   82.97%   84.88%   +1.90%     
==========================================
  Files           7        8       +1     
  Lines         141      172      +31     
==========================================
+ Hits          117      146      +29     
- Misses         24       26       +2     

Impacted Files	Coverage Δ
lib/src/interaction.dart	93.02% <ø> (ø)
lib/src/errors.dart	7.14% <33.33%> (+7.14%)	⬆️
lib/src/matchers.dart	100.00% <100.00%> (ø)

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