fix(ext typings): remove some accessibility modifiers from Component

[davwheat]

Changes proposed in this pull request:

At the moment, our Component class has various protected attributes and methods. Sometimes, we need to override/extend these methods (initAttrs) or to use these attributes (this.$/this.attrs) within the body of extend or override.

This presents an issue, as our typings pretend these don't exist outside of the class and its subclasses, so show up as errors in TS.

This PR removes those accessibility modifiers to prevent these TS errors.

Reviewers should focus on:

I don't see this as an issue since these are regularly used outside of the classes anyway in JS-based extends/overrides.

Necessity

• Has the problem that is being solved here been clearly explained?
• If applicable, have various options for solving this problem been considered?
• For core PRs, does this need to be in core, or could it be in an extension?
• Are we willing to maintain this for years / potentially forever?

Confirmed

• Frontend changes: tested on a local Flarum installation.
• Backend changes: tests are green (run composer test).
• Core developer confirmed locally this works as intended.
• Tests have been added, or are not appropriate here.