Implement MergePatch

[markcowl]

Description is here: https://gist.github.com/markcowl/5ad4aa0bfac2b3a981cc35e1e00ca98a

• Add new JS API types
  • MergePatchModel
  • MergePatchModelProperty
  • MergePatchProperties
• Implement @withMergePatch decorator
  • Implement mutator to create new types using MergePatch + visibility transform
  • Mark transformed types with the mergePatchSource property
  • Mark transformed properties with the mergePatchProperties property
  • Some mechanism for resolving name clashes
  • Tests
• Implement js APIs
• Add TypeSpec types
• Update getHttpOperation to skip optionality transform when a MergePatch type is the body
• Implement linting rules for proper use of MergePatch
• Tests
• Language basics doc for MergePatch
• (Later) Add MergePatch scenarios to lib:http

[markcowl]

est: 21