Enable TS type-only class fields by switching TS Babel from preset to transform plugin

Created by: pekala

Fixes https://github.com/facebook/create-react-app/issues/8918

Type-only class fields using the declare keyword were added by TS 3.7. CRA currently fails when it encounters them with the following error:

TypeScript 'declare' fields must first be transformed by @babel/plugin-transform-typescript.
If you have already enabled that plugin (or '@babel/preset-typescript'), make sure that it runs before any plugin related to additional class features:
 - @babel/plugin-proposal-class-properties
 - @babel/plugin-proposal-private-methods
 - @babel/plugin-proposal-decorators

Declare fields are useful when using React context in class components, e.g.:

class MyComponent extends Component<MyProps> {
    static contextType = MyContext
    declare context: React.ContextType<typeof MyContext>

This PR switches from Babel TS preset to Babel TS transform plugin in order to run the babel transform before @babel/plugin-proposal-class-properties. It also switches the support for declare fields by using the allowDeclareFields option on the plugin. This will be the default for Babel 8, but for now, we need to use the option.

The preset to plugin switch should have no other effect since the babel TS preset wraps only one plugin and provides little more than the transform plugin itself.