feat: support embed struct fields without tstype: ",extends"

[mtt0]

Backgroud

For use case of we must use embed struct field's (anonymous field's), but the json/yaml/... tag is not inline.

If we got

type Deployment struct {
    v1.TypeMeta `json:",inline"  tstype:",extends"`
    v1.ObjectMeta `json:"metadata"`
}

the serialized string be like

{
  "apiVersion": "v1",
  "kind": "Deployment",
  "metadata": {
    "name": "test"
  }
}

In this case, I need metadata json object, and v1.ObjectMeta's methods which Deployment type needs to compose of.

Currently tygo will ignore anonymous field. And tstype:",extends"'s behavior dosen't meet my needs.
Golang uses composition instead of inheritance, inline and tystype:",extends" match exactly in meaning and behavior, while anonymous fields behave differently when serializing and deserializing.

This pr is trying to support embed struct fields (anonymous field) in such case.

Example

package embed

import bookapp "github.com/gzuidhof/tygo/examples/bookstore"

// TokenType Built-in type alias
type TokenType string

// Reference struct type
type Reference struct {
	Foo string `json:"foo"`
}

type Base struct {
	ID string `json:"id"`
}

type StructEmbed struct {
	Base           `json:",inline" tstype:",extends"` // embed struct with `tstype:"extends"`
	TokenType      `json:"tokenType"`                 // built-in type field without `tstype:"extends"`
	Reference      `json:"reference"`                 // embed struct without `tstype:"extends"`
	OtherReference Reference                          `json:"other_reference"`
	Bar            string                             `json:"bar"`
	bookapp.Book   `json:"book"`                      // embed external struct without `tstype:"extends"`
	Chapter        bookapp.Chapter                    `json:"chapter"`
}

With tygo.yaml:

packages:
  - path: "github.com/gzuidhof/tygo/examples/embed"
    fallback_type: unknown
    type_mappings:
      bookapp.Book: "bookapp.Book"
      bookapp.Chapter: "bookapp.Chapter"
    frontmatter:
      | # We can define some additional text to put at the start of the file.
      import * as bookapp from "../bookstore"

Result:

// Code generated by tygo. DO NOT EDIT.
import * as bookapp from "../bookstore"

//////////
// source: embed.go

/**
 * TokenType Built-in type alias
 */
export type TokenType = string;
/**
 * Reference struct type
 */
export interface Reference {
  foo: string;
}
export interface Base {
  id: string;
}
export interface StructEmbed extends Base {
  tokenType: TokenType; // built-in type field without `tstype:"extends"`
  reference: Reference; // embed struct without `tstype:"extends"`
  other_reference: Reference;
  bar: string;
  book: bookapp.Book; // embed external struct without `tstype:"extends"`
  chapter: bookapp.Chapter;
}

[gzuidhof]

@0x-buidl would you perhaps be able to review this?

Of course if you're not available, let me know and I'll happily dive into it :)

[0x-buidl]

Yeah, i'll make a review

[0x-buidl]

@gzuidhof this PR is good too go.

[gzuidhof]

Thank you for the contribution @mtt0, and thank you @0x-buidl for the review :)