Backport of 'Make Rspack output independent of the checkout path' into 19.5

What does this MR do and why?

Backports the two commits of !262551 (merged) so the Rspack assets of 19.5 no longer depend on the directory they are built in:

• Keep checkout paths out of Vue 3 lane imports. The Rspack infection loader rewrites each ?vue3 import to a path relative to the importing file instead of the target's absolute path. Rspack sizes modules by the loader's output and splitChunks places them by size, so the chunk layout no longer changes with the checkout path. Adds spec/frontend/config/vue3migration/plugins/rspack_loader_spec.mjs.
• Mangle Rspack output without char frequency. Sets SWC's mangle.disableCharFreq, so the runtime's variable names, and with them its file name, no longer depend on the build directory (https://github.com/web-infra-dev/rspack/issues/14089).

Both commits apply to 19-5-stable-ee without conflicts. This branch locks the same Rspack as master (2.2.8). webpack output is unchanged.

In production, Rails serves Rspack chunks only when the rspack_production_assets feature flag is on, or its per-user companion rspack_production_assets_opt_in, unless ENABLE_RSPACK overrides them (rspack_enabled?). On 19-5-stable-ee both flags are type gitlab_com_derisk with default_enabled: false, so by default 19.5 keeps serving the webpack build.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• This MR is backporting a bug fix, documentation update, or spec fix, previously merged in the default branch.
• The MR that fixed the bug on the default branch has been deployed to GitLab.com (not applicable for documentation or spec changes).
• The MR title is descriptive (e.g. "Backport of 'title of default branch MR'"). This is important, since the title will be copied to the patch blog post.
• Required labels have been applied to this merge request
  • severity label and bug subtype labels (if applicable)
  • If this MR fixes a bug that affects customers, the customer label has been applied.
• This MR has been approved by a maintainer (only one approval is required).
• Ensure the E2E Omnibus GitLab EE pipeline (triggered by thee2e:test-on-omnibus-ee job) has succeeded. If it has failed (or shows warnings), investigate the failures. If you determine the failures are unrelated, you may proceed. If you need assistance investigating, request help in the #s_developer_experience Slack channel to confirm the failures are unrelated to the merge request.

Note to the merge request author and maintainer

If you have questions about the patch release process, please:

• Refer to the patch release runbook for engineers and maintainers for guidance.
• Ask questions on the #releases Slack channel (internal only).
• Once the backport has been merged, the commit changes will be automatically deployed to a release environment that can be used for manual validation. See after merging runbook for details.