fix: port AppModal component to harvester source code

[a110605]

Summary

Previous PR port AppModal component from Rancher/shell to harvester shell. But the appModal was introduced after Rancher 2.9+ which causes it doesn't exist on Rancher 2.8.

This PR port AppModal component to harvester source code.

PR Checklist

• Is this a multi-tenancy feature/bug?
  • Yes, the relevant RBAC changes are at:
• Do we need to backport changes to the old Rancher UI, such as RKE1?
  • Yes, the relevant PR is at:
• Are backend engineers aware of UI changes?
  • Yes, the backend owner is:

Related Issue #
harvester/harvester#7147

Screenshot/Video

1. Import harvester into Rancher 2.8 or 2.9
2. Pull this PR locally and run `yarn build-pkg harvester && yarn serve-pkgs`
3. Change 
   - ui-plugin-index setting to http://127.0.0.1:4500/harvester-1.4.0/harvester-1.4.0.umd.min.js
   - ui-source to external
4. Refresh

Test on Rancher 2.9

2.9.2.mov

Test on Rancher 2.8

2.8.5.mov

[a110605]

The original mountTo is id=modals , but on Rancher 2.8, there is no id=modals in DOM. ( Because it's introduced via shell/public/index.html

Change mounTo #__layout seems no harm

[houhoucoop]

LGTM