[2.x] fix(deck): name the parts of a filter it can't use - #5459

Reported on Discuss: https://discuss.flarum.org/d/40038-introducing-deck-follow-every-conversation-you-care-about-live/4 (no issue)

Changes proposed in this pull request:
When a custom filter column's query has a part that isn't a discussion filter, Deck said "Use filters only, such as tag:support is:unread. Plain words aren't supported.", which led a member to think combined filters weren't supported (they are; the part it refused was group:, then a users-only filter). It now names the parts it can't use, e.g. "group:mods isn't a filter for discussions. Use filters such as tag:support or is:unread.", in both the Add column form and the Filters window, and the message clears once the query is edited.

For extensions, a column field's invalidText can now be a function of what was typed.

Reviewers should focus on:

• Unknown parts are worked out from the registered gambits (app.search.gambits.match), so extensions' filters count as known.
• Runs of spaces or tabs in a pasted query aren't reported as parts.

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.
• Frontend changes: tests are green (run yarn test in js/).
• Frontend changes: tests have been added, or are not appropriate here.
• Backend changes: tests are green (run composer test).
• Backend changes: tests have been added, or are not appropriate here.
• Where applicable, changes are suitable for all supported database drivers (MySQL, MariaDB, PostgreSQL, SQLite).
• Core developer confirmed locally this works as intended.
• The description above is written by me and describes what this pull request actually does.

Required changes:

• Related documentation PR: docs(deck): invalidText can be a function docs#710