feat: SlickVPN Support (#961)

- `internal/updater/html` package
- Add unit tests for slickvpn updating code
- Change shared html package to be more share-able
- Split html utilities in multiple files
- Fix processing .ovpn files with prefix space

Authored by @Rohaq 
Co-authored-by: Quentin McGaw <quentin.mcgaw@gmail.com>

.github/ISSUE_TEMPLATE/bug.yml

@@ -54,6 +54,7 @@ body:
         - PrivateVPN
         - ProtonVPN
         - PureVPN
+        - SlickVPN
         - Surfshark
         - TorGuard
         - VPNUnlimited