[tags] Fix wrong tag input width for CJK characters

[YUCLing]

Fixes #0000

Changes proposed in this pull request:
Add a text measurement utility function in TagSelectionModal that is based on canvas's measureText. And make input width prefer this instead of ch unit.

The reason of ch unit doesn't work:

Equal to the used advance measure of the “0” (ZERO, U+0030) glyph found in the font used to render it. (The advance measure of a glyph is its advance width or height, whichever is in the inline axis of the element.)

Reviewers should focus on:

Do you think should we make the measurement utility a globally available function?

Screenshot

Before

After

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.

Required changes:

• Related documentation PR: (Remove if irrelevant)