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>

internal/configuration/settings/openvpnselection.go

@@ -82,6 +82,9 @@ func (o OpenVPNSelection) validate(vpnProvider string) (err error) {
 			case providers.Protonvpn:
 				allowedTCP = []uint16{443, 5995, 8443}
 				allowedUDP = []uint16{80, 443, 1194, 4569, 5060}
+			case providers.SlickVPN:
+				allowedTCP = []uint16{443, 8080, 8888}
+				allowedUDP = []uint16{443, 8080, 8888}
 			case providers.Wevpn:
 				allowedTCP = []uint16{53, 1195, 1199, 2018}
 				allowedUDP = []uint16{80, 1194, 1198}