Add support for simple Identifier replacement in type_mappings config

[KONFeature]

Support the replacement of simple ast.Ident via type mappings, thus providing support for type alias replacement on the config level.

Replacement similar to the previous ast.SelectorExpr, just the longtype is built with id_{identifierName}.

Could be very useful in projects with a lot of type aliases, for now stuck with adding a lot of tstypes tags

Example

Go:

type DurationString = string

type CacheConfig struct {
	Key string        `json:"key"`
	Ttl DurationString `json:"ttl"`
}

Config:

packages:
- path: "..."
    type_mappings:
      id_DurationString: "TsDurationString"
    frontmatter:
      export type TsDurationString = `${number}ms` | `${number}s` | `${number}m` | `${number}h`;

Output:

// Code generated by tygo. DO NOT EDIT.
export type TsDurationString = `${number}ms` | `${number}s` | `${number}m` | `${number}h`;
//////////
// source: alias.go

/**
 * Represent a duration that would be parsed with smth like `time.ParseDuration(...)`
 */
export type DurationString = string;
export interface CacheConfig {
  key: string;
  ttl: TsDurationString;
}