TypeScript codegen backend

[cometkim]

See #6196 (comment) for context.

I tried to implement all the features we need, including those not supported by genType today.

Currently, it is somewhat complex to support both .ts and .d.ts outputs. Eventually, it could simplify the codegen process by unifying the codebase after deprecating genType and by organizing the context and config structures.

Config

For dts:

{
  "sources": "src",
  "package-specs": [
    {
      "module": "esmodule",
      "in-source": true,
+     "dts": true
    }
  ]
}

For TypeScript
(config schema would be changed)

{
  "sources": "src",
+ "language": "typescript",
  "package-specs": [
    {
      "module": "esmodule",
      "in-source": true
    }
  ]
}

Attributes

• @as: Rename the generated types.

  type user // => interface user
  
  @as("User")
  type user // => interface User

• @external("TypeName"): Use external (global) type

• @external(("package-name", "Name", true)): named type import

• @external(("package-name", "default", true)): default import as type

• @external(("package-name", "*", true)): namespace import as type

• @opaque: Force the type to be opaque

  type t 
  // abstract types will automatically be opaque
  
  type t<'a, 'b> = 'b
  // opaque type because 'a is phantom type
  
  @opaque
  type t = string
  // branded string type

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commit: 2f90e46