Ast: Distiguish class/object methods with ordinary function expressions in FunctionType

[KermanX]

The FunctionType enum already distinguishes function declarations and expressions. However, class methods and object methods are also considered to be function expressions, although they are different in some ways. For example:

const object = {
  fn: function(){},
  method(){}
}
object.fn.prototype     //=> {}
object.method.prototype //=> undefined

A possible solution is adding MethodFunction to the FunctionType enum.

[overlookmotel]

Similar to: oxc-project/backlog#142

[Boshen]

This conforms to estree. The caller will need to figure out whether it's a function inside a class / object or not.

[overlookmotel]

Personally, I don't think we need to conform to ESTree. Serialization can take care of translating our AST to ESTree-compatible AST. But I think it's fine to close this issue, as oxc-project/backlog#142 is sufficiently similar.