[interop] Add Support for Enums

[nikeokoronkwo]

Closes #393

This PR adds support for enumerations in the Dart JS Interop Interface Gen project.
Enums in TS with the enum keyword are mapped to (constant) Dart extension types with the values being static properties.
Therefore:

export declare enum Direction {
  Up = 0,
  Down = 1
}

Turns into:

extension type const Direction._(num _) {
  static const Direction Up = Direction._(0);
  static const Direction Down = Direction._(1);
}

This enum pattern is used to represent TS Syntax Kinds in the generator.
Although the vast majority of enums declared with the enum keyword are homogenous with defined number or string values:

• If an enum value is computed and does not have a default value, it is declared external (and the rep type becomes a JS-compatible rep type)
• If an enum is heterogenous, then the rep type becomes JSAny
• If the rep type of an enum is a JS type, then the enum properties can no longer be constant due to literal conversions.

This PR also supports homogenous union types as enums (i.e "north" | "south" | "east" | "west"), translating them to become enum declarations.

[srujzs]

Looking at declaration, if this is always going to be an Enum, can we call it HomogenousEnumType?

[nikeokoronkwo]

Or maybe UnionEnumType? I feel maybe calling it HomogenousEnumType would confuse it as being directly related to an EnumDeclaration when it only is in reference. (I might be totally wrong on this however, but what do you think?)