Remove the tsconfig option `useDefineForClassFields` and fix relevant TS files

[Goodwine]

The recent update on gts included new settings for tsconfig.json. Specifically, it turned on useDefineForClassFields. And this changes the Typescript behavior in how/when class properties get initialized.

Related: microsoft/TypeScript#45995 (comment)

The task for this Issue with regards to sass-parser is to find all properties initialized with !, and replace them with a declare statement, for example:

class X {
  // BAD:
  _foo!: string;
}

class X {
  // GOOD:
  declare _foo: string;
}

Otherwise, the observed behavior for our PostCSS subnode classes is that all the values get reset after they get initialized to their "default" values.

[Goodwine]

some properties may be defined with ? instead of !. So I guess the thing to do here is to check everything thoroughly