Allow extensions to modify MIN_SEARCH_LENGTH in Search component

[dsevillamartin]

Fixes #3129

Changes proposed in this pull request:

• Use this.constructor instead of accessing the parent class directly to allow subclasses to modify MIN_SEARCH_LEN static

Reviewers should focus on:

• Perhaps clarifying wording of docblock

Screenshot
No user facing changes in core itself. This is helpful for fof/merge-discussions, changing the min search length to 1:

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.

[askvortsov1]

I've thught about this one a bit, and I think that once we have #3127 (review) and adding settings defaults has much less overhead, we should put this into a setting instead. We don't need an admin UI for it necessarily, but having it implemented as a setting would make customization easier.

[dsevillamartin]

@askvortsov1 This is not intended to modify the min search length of the navbar search bar - that can be done already by modifying Search.MIN_SEARCH_LEN through any JS code.

This is for extensions that extend the component and want a different minimum search length for their own custom search bar. The logic for searching is all cluttered in oncreate and event hooks and so requires a lot of code to be copy & pasted to change that as of now.

I agree a setting would be nice, but that wouldn't address different search components used by extensions and their intended use cases.