[6.x] Support for namespaced tags

[marcorieser]

This PR adds opt-in namespace support for tags, analogous to how views are namespaced (vendor::view):

{{ juicebox::my_tag:method }}

Why

Tag handles are currently global. Two addons registering a tag with the same handle silently collide (last one wins), and there's no way to explicitly address a specific addon's tag.

Usage

Addons opt in by setting a property on their service provider:

class ServiceProvider extends AddonServiceProvider {
    protected $tagNamespace = 'juicebox';
}

When set, the addon's tags (and their aliases) are registered only under the namespace — {{ juicebox::my_tag }} — and no longer under their bare handles. Since the property defaults to null, existing addons are unaffected.

The underlying mechanism is generic: RegistersItself::register() now accepts an optional namespace, so any tag class can be registered namespaced manually:

MyTag::register('juicebox');

Namespaced tags work across all three surfaces:

• Antlers: {{ juicebox::my_tag:method }}, including paired tags, self-closing tags, and interpolations in conditions
• Blade components: <s:juicebox::my_tag:method />
• Fluent: Statamic::tag('juicebox::my_tag:method')

Notes

• Non-breaking: behavior only changes when an addon explicitly sets $tagNamespace.