Mimic TS class property behavior change

[alangpierce]

Tracked/discussed in microsoft/TypeScript#27644

At the moment, Sucrase always transpiles class fields into constructor assignments, and class fields without initializers are removed:

class Foo {
  a: number;
  b: number = 2;
}

becomes:

"use strict";
class Foo {constructor() { Foo.prototype.__init.call(this); }
  
  __init() {this.b = 2}
}

In the future, we'll need to change the behavior to "define" semantics, and soon enough it'll be possible to just drop class field transpile completely, since it's supported in Node 12 but not Node 10. When dropping class field support, we'll need to support declare for fields, so that this:

class Foo {
  a: number;
  b: number = 2;
  declare c: number;
}

becomes this:

class Foo {
  a;
  b = 2;
  
}

Right now there isn't a way to disable class field transpile, and the behavior with transpiled class fields isn't totally clear. A safe first step is to at least parse and remove declare fields.