fix(menu): Use package import for sp-menu-divider registration

[Govind-gupta75]

Menu.ts registers via a relative side-effect import. In build systems that alias @spectrum-web-components/* packages (for example UXP wrappers), that relative path can resolve to a different module instance than the package export consumers use, causing duplicate custom element registration.

This follows the same fix applied in #3225 for MenuGroup, DialogBase, DialogWrapper, and Table.

Before

import '@spectrum-web-components/icons-ui/icons/sp-icon-arrow500.js';
import '../sp-menu-divider.js';

After

import '@spectrum-web-components/icons-ui/icons/sp-icon-arrow500.js';
// Leveraged in build systems that use aliasing to prevent multiple registrations: https://github.com/adobe/spectrum-web-components/pull/3225
import '@spectrum-web-components/menu/sp-menu-divider.js';

Problem
When extending SWC components in a wrapper layer and aliasing package paths (for example '@spectrum-web-components/menu': '@swc-uxp-internal/menu'), a relative import like ../sp-menu-divider.js may load the registration file from a different module graph than @spectrum-web-components/menu/sp-menu-divider.js. That can trigger:

NotSupportedError: Failed to execute 'define' on 'CustomElementRegistry':
the name "sp-menu-divider" has already been used with this registry

Motivation and context
Aligns Menu.ts with MenuGroup.ts, which already uses @spectrum-web-components/menu/sp-menu.js with the PR #3225 comment.
Ensures aliased/webpack-based builds register sp-menu-divider once.
No API or visual behavior change; import path only.
Related issue(s)
Related: #3225

Screenshots (if appropriate)

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Coverage: 96.24%

Details

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Coverage Stats

Relevant Lines:	39111
Covered Lines:	37840
Line Coverage:	96.75%
Relevant Branches:	6453
Covered Branches:	6011
Branch Coverage:	93.15%
Branches in Coverage %:	Yes
Coverage Strength:	458.12 hits per line

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