📎 Refactor/fix JsParserSettings for better override behavior

[dyc3]

Description

We've recently found that overrides specified in configuration caused json parser settings to be overwritten, even when the override didn't specify those options. Found in #3260, fixed in #3272, see also #3273.

This task is to investigate and fix any similar issues for JsParserSettings.

[Conaclos]

See also #3183

We have a similar issue with jsxRuntime.

[Sec-ant]

I created #3317 @dyc3, I think it already addressed this issue. But there's a bigger refactoring task behind. Feel free to get involved. There're some discussions at #3310

[Conaclos]

Is this issue addressed by #3317?

[Sec-ant]

Is this issue addressed by #3317?

It will be, but given the scope of the refactoring, addressing merge conflicts has become more and more difficult with scattered commits, so I squashed all the commits. And that PR is not ready to be merged yet because I can't find time to work on it recently.