Refactor config parser with json_serialization

[lcdsmao]

What does this change?

• Use json_serialization to model config, so we don't need to interact with the YAML map directly.
  • This also enables us to easily add/edit the config option.
• Merge user's pubspec.yaml into a default config before parsing, so we don't need to handle null case.

PS

We can not use build_runner directly inside a builder project.
So we need to create another project (_internal) for running build_runner.
See: #dart-lang/build#2982

What is the value of this and can you measure success?

Pass test

[codecov]

Codecov Report

Merging #76 (cd2335c) into main (c2cb180) will decrease coverage by 1.23%.
The diff coverage is 95.41%.

@@            Coverage Diff             @@
##             main      #76      +/-   ##
==========================================
- Coverage   99.34%   98.11%   -1.24%     
==========================================
  Files          17       16       -1     
  Lines         458      477      +19     
==========================================
+ Hits          455      468      +13     
- Misses          3        9       +6     

Impacted Files	Coverage Δ
lib/src/settings/pubspec.dart	83.87% <83.87%> (ø)
lib/src/flutter_generator.dart	93.33% <95.65%> (-6.67%)	⬇️
lib/src/generators/assets_generator.dart	100.00% <100.00%> (ø)
lib/src/generators/colors_generator.dart	98.21% <100.00%> (-0.04%)	⬇️
lib/src/generators/fonts_generator.dart	100.00% <100.00%> (ø)
lib/src/settings/config.dart	100.00% <100.00%> (ø)
lib/src/settings/pubspec.g.dart	100.00% <100.00%> (ø)
lib/src/utils/cast.dart

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[wasabeef]