Allow importing types from kibana/{public,server} for x-pack TypeScript

[joshdover]

Summary

This change adds the kibana/public and kibana/server import paths for TypeScript code in x-pack. This aligns with the aliases provided in OSS TypeScript code.

There is some overlap between this change and this discussion in #40446. I propose we go ahead and add this now because:

• Importing types from Core is a very common usecase in the New Platform. Not having to do very long relative imports to the top of the repo's file tree is convenient.
• If we move forward with the propose change in An alternative to relative imports for local source code #40446, changing these imports from kibana/public to @kibana/core/public or similar should be easy.

Checklist

Use strikethroughs to remove checklist items you don't feel are applicable to this PR.

• This was checked for cross-browser compatibility, including a check against IE11
• Any text added follows EUI's writing guidelines, uses sentence case text and includes i18n support
• Documentation was added for features that require explanation or tutorials
• Unit or functional tests were updated or added to match the most common scenarios
• This was checked for keyboard-only and screenreader accessibility

For maintainers

• This was checked for breaking API changes and was labeled appropriately
• This includes a feature addition or change that requires a release note and was labeled appropriately

[elasticmachine]

Pinging @elastic/kibana-platform

[elasticmachine]

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