feat: search UI/UX revamp

[SychO9]

Part of the roadmap

Changes proposed in this pull request:
This pull request implements the design work made by @iamdarkle (https://discuss.flarum.org/d/33443-flarum-20-uiux-improvements-for-enhanced-search-experience/) to improve the searching experience.

The obvious visual changes include:

• Stripping the search input of its dropdown results (and related logic).
• Introducing a new Search modal component (lazy loaded).

Some key technical changes/additions are:

• The abstraction of the autocomplete logic used in mentions and emoji, into a new AutocompleteReader, we use this here for the gambit autocompletion.
• The introduction of a GambitsAutocomplete utility which makes use of the AutocompleteReader.
• The introduction of a GambitsAutocompleteDropdown used for independent inputs (like the admin users page), unrelated to the SearchModal.
• The API for the gambit interface has been changed and improved to allow structuring suggestions. New BooleanGambit and KeyValueGambit classes have been added to reduce the boilerplate of implementing gambits.
• Gambits can now be localized (check the .yml files)

Screenshot
screen-capture (19).webm

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.