lint: Add linter that finds unused imports in php classes

[Monviech]

Idea when thinking about this: #10052

Example output:

root@opn-dev-02:/src/git/core # make lint-import

src/www/firewall_rule_lookup.php: warning: stale import use OPNsense\Firewall\Filter
src/opnsense/mvc/tests/app/models/OPNsense/Base/FieldTypes/PortFieldTest.php: warning: stale import use OPNsense\Firewall\Util
src/opnsense/mvc/script/run_validations.php: warning: stale import use OPNsense\Core\Config
src/opnsense/mvc/app/library/OPNsense/Base/UIModelGrid.php: warning: stale import use OPNsense\Core\Type
src/opnsense/mvc/app/library/OPNsense/Auth/LDAPTOTP.php: warning: stale import use OPNsense\Core\Config
src/opnsense/mvc/app/library/OPNsense/Auth/Voucher.php: warning: stale import use OPNsense\Core\Config
...

[Monviech]

Giving this an the linter PR to @fichtner for later to decide and adjust as needed.

[fichtner]

that's a good idea... we should improve the pattern matching but otherwise this is great, thanks