Add support for declare syntax for TS class fields

[alangpierce]

See https://www.typescriptlang.org/docs/handbook/release-notes/typescript-3-7.html#the-usedefineforclassfields-flag-and-the-declare-property-modifier . As part of switching to fully spec-compliant class fields, a TS project will need to look for non-initialized class fields and make sure that the behavior is correct when using define. For example, with this code:

class A {
  foo: number | string = 3;
}

class B extends A {
  foo: string;
}

TypeScript with useDefineForClassFields: true gives the error TS2612: Property 'foo' will overwrite the base property in 'A'. If this is intentional, add an initializer. Otherwise, add a 'declare' modifier or remove the redundant declaration.

The typical fix is to change the class definition to:

class B extends A {
  declare foo: string;
}

But Sucrase currently fails to parse the declare syntax, so that's an important prerequisite step.