Add tag() action for CollectionService tags

[BitwiseAndrea]

Summary

Adds a built-in vide.tag() action that wraps Instance:AddTag() / Instance:RemoveTag() so CollectionService tags can be applied directly through create() props, alongside the existing changed() helper.

create "Frame" {
    tag("Clickable"),
    tag({ "Draggable", "Highlightable" }),
    tag(function() return "state-" .. state() end), -- reactive
}

• Accepts a static string, a { string }, or a reactive source returning either.
• The reactive variant diffs the previous and new tag sets, so tags present in both are left untouched; only the delta is added/removed.
• All tags added by the action are removed when the surrounding scope is destroyed (via cleanup).
• Multiple tag() actions compose on the same instance.
• Default action priority of 1, matching changed().

Loosely modeled after react-luau's Tag handling, but exposed as an action to fit Vide's existing patterns (rather than a magic prop key). Uses the newer Instance:AddTag / RemoveTag methods directly rather than going through CollectionService.

Changes

• src/tag.luau — new action implementation.
• src/lib.luau — exports vide.tag.
• test/mock.luau — adds AddTag / RemoveTag / HasTag / GetTags to the mock Instance so the action is testable outside Studio.
• test/tests.luau — 8 new cases covering single/multi/static/reactive tags, scope-destroy cleanup, and composition.
• docs/api/creation.md — new tag() API reference section.
• docs/tut/crash-course/12-actions.md — closing note pointing readers at the built-in changed() and tag() helpers.
• CHANGELOG.md — entry under Unreleased / Added.

Test plan

• luau test/tests.luau — 130/130 cases pass, including the 8 new tag() cases.
• Manual smoke test in Studio against a real Instance (Add/Remove/Has/GetTag).

Made with Cursor

[isarsindri]

Just repeating what Centau said from another PR
Vide is designed to (mostly) function in vanilla luau, outside of Roblox. Being dependent on (whateverService) makes this another feature that can only be used in Roblox.