Group the Matrix block action menu’s “Add … above” items by entry type group

[fabianhaef]

Description

Fixes #17594.

In the blocks view mode, each Matrix block’s action menu appends one “Add {type} above” item per entry type, in one flat list — also reported in #17594. We have Matrix fields with 35+ entry types, and the menu becomes a long scrolling list where the only way to find a type is to read every row.

The field’s “+” button already solves this: create-button.twig groups the same entry types by their entry type group and shows a search input above five types. This PR brings the block action menu in line with it:

1. Group the “Add {type} above” items by entry type group — same collect().groupBy() and {hr, padded: false} / {heading, padded: false, items} structure as create-button.twig. Fields with a single group (or no groups set) render exactly as before.
2. Show a search input above five entry types (withSearchInput: not staticEntries and entryTypes|length > 5), same threshold as the create button. The search filters all menu items (Collapse, Delete, …), like other searchable disclosure menus — it’s a separate commit so it can be dropped if that’s not wanted.

Drive-by: the trailing {hr: true} is now only added when add items follow, removing a dangling separator at the end of the menu for static blocks (e.g. revision view).

Testing

• 20 entry types across 6 groups: headings and separators render in field-settings order; activating a grouped item still inserts the block above it; arrow-key navigation skips headings.
• Searching narrows the menu and hides emptied group headings; activating a filtered item works.
• Single group / no groups set: unchanged flat menu.
• Revision view (staticEntries): no add items, no search input, no trailing separator.
• maxEntries reached: DisclosureMenu.updateVisibility() hides the emptied groups including their headings.

[fabianhaef]

There’s a follow-up discussion for #17594 in #17600 with a few community suggestions — how this PR relates to them:

• Nested submenus per group (Notion-style): Craft’s disclosure menus don’t support nested submenus today, so this PR groups inline with headings — the same pattern create-button.twig uses. If submenu support lands later (e.g. via the Display grouped “New entry” buttons as a button group #17731 direction), the grouped structure here maps onto it 1:1.
• “Delete” being hard to reach: the add items sit below the standard actions, so grouping doesn’t change Delete’s position — and the search input (second commit) gets you to any item by typing a few letters.
• Keeping the flat view: fields without entry type groups render exactly as before, so the flat list stays available by simply not assigning groups. A dedicated per-field toggle would be a small addition if you’d prefer the choice to be explicit.