feat(embind): add a way to register enum values as plain string

[FelixNumworks]

Fix #24324
Fix #19387
Fix #18585

This PR adds a bool asString parameter to embind enum declaration

enum_<MyEnum>("MyEnum"); // Defaults to false
enum_<MyEnum>("MyEnum", true); // asString

If asString is true, the enum is greatly simplified, since the name of a value is equivalent to the value itself.

MyEnum.ONE === "ONE" // true

myFuncTakingEnumParam(MyEnum.ONE);
myFuncTakingEnumParam("ONE");

We go from

export interface MyEnumValue<T extends number> {
  value: T;
}
export type MyEnum = MyEnumValue<0>|MyEnumValue<1>|MyEnumValue<2>;

interface EmbindModule {
  MyEnum: {valueOne: MyEnumValue<0>, valueTwo: MyEnumValue<1>, valueThree: MyEnumValue<2>};
};

to

export type MyEnum = 'valueOne'|'valueTwo'|'valueThree'

interface EmbindModule {
  MyEnum: {valueOne: 'valueOne', valueTwo: 'valueTwo', valueThree: 'valueThree'};
};

This doesn't conflict with current implementation of enums, as the bool default value is false

This is my first PR to emscripten, so I'm sorry in advance if it contains some obvious flaws ...
I really hope this gets into the main code, as it would really simplify enums handling

[brendandahl]

Since this doesn't have any shared code let's split it out into it's own function _embind_register_enum_string or something. I'm guessing users will only use one way other other, so then if it's a totally seperate function it will be easier to do dead code elimination.

[brendandahl]

Same here, split into its own function. I think you'll have to store on the enum class in c++ whether it's a string or not and call the correct register function from there.