fix PreferenceChangeEvent<T> type

[paul-marechal]

What it does

Improves the PreferenceChangeEvent<T> type to properly reflect how it is meant to be used. I also changed the few places where the type was given wrong information. See commit message for details.

How to test

You can inspect the type of event.newValue in places where we check event.preferenceName and make sure that instead of inferring an union of all values for all preference names, we now have the proper type.

Review checklist

• as an author, I have thoroughly tested my changes and carefully followed the review guidelines

Reminder for reviewers

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[paul-marechal]

I noticed this oddity while reviewing something else, figured we could fix it.

[vince-fugnitto]

The changes look good to me, and fix the issue I've noticed when dealing with the onPreferenceChanged event among other things.

We now have the correct typing when doing the following and use the event directly:

this.preferences.onPreferenceChanged(event =>  {
    if (event.preferenceName === 'git.editor.dirtyDiff.linesLimit') {
        const value = event.newValue; // we correctly have a number.
    }
});

rather than the workaround for master:

this.preferences.onPreferenceChanged(event =>  {
    if (event.preferenceName === 'git.editor.dirtyDiff.linesLimit') {
        const newValue = event.newValue; // newValue does not return the proper typings (returns all typings from the schema).
        const value = this.preferences['git.editor.dirtyDiff.linesLimit']; // workaround to get the proper newValue.
    }
});