[Fix #1694] Add projectile-invalidate-cache-all command

[takeokunn]

Summary

Add a new interactive command projectile-invalidate-cache-all that clears the file cache for all known projects at once — both in-memory hash tables and on-disk persistent cache files.

This resolves #1694 where projectile-find-file-in-known-projects shows stale file listings (deleted files still appear, new files missing) because projectile-invalidate-cache only operates on a single project.

Closes #1694

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• The commits are consistent with our contribution guidelines
• You've added tests (if possible) to cover your change(s)
• All tests are passing (eldev test)
• The new code is not generating bytecode or M-x checkdoc warnings
• You've updated the changelog (if adding/changing user-visible functionality)
• You've updated the docs (when adding new project types, configuration options, commands, etc)

Thanks!