Add functions:kits:uninstall command (#10981)

Files changed:
src/config.ts: Extends some existing helper functions for filesystem operations with project root relative paths to also have options for directory-level operations
src/commands/functions-kits-uninstall.ts‎: Implements the actual command
src/commands/index.ts: Registers the command with the CLI, gated by the kits experiment

Scenarios tested:
Instance deletion (not last instance), instance deletion (last instance triggering kit deletion), instance deletion (not yet deployed) direct kit deletion.
Conservative filesystem case tested by manually modifying firebase.json to strip out the packageSource field