Closes #1001

Add a new command that lists all files under the project root using
a generic file listing command (fd or find), bypassing .gitignore,
.projectile, and other ignore mechanisms. This is useful when you
need to find files that are normally excluded from the project.

- Use VCS-appropriate command in projectile-get-sub-projects-files
  instead of hardcoding projectile-git-command.
- Add projectile-hg-ignored-command and projectile-svn-ignored-command
  defcustoms so ignored-file filtering works for hg and svn projects
  in native/hybrid indexing mode.

seq-union requires Emacs 28.1+ and seq-keep requires Emacs 29.1+.

seq-union is not available in Emacs 27's built-in seq.el.
Use (seq-uniq (append ...)) which achieves the same result.

The fboundp guard was removed but vc-git is only loaded at runtime,
so the byte-compiler needs a declare-function to avoid a warning.

seq-keep was added in Emacs 29.1 and is not provided by the compat
package.  Revert to (delq nil (mapcar ...)) which works on all
supported Emacs versions.

This function was defined but never called anywhere in the codebase.

Use seq-keep (available via compat 30) for the filter-map pattern
in projectile-normalise-paths, projectile-rgrep-default-command,
and projectile-open-projects.

Use the non-destructive seq-uniq instead of delete-dups across
9 call sites for consistency with the seq.el migration.