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/constants/providers/providers.go

@@ -19,6 +19,7 @@ const (
 	Privatevpn            = "privatevpn"
 	Protonvpn             = "protonvpn"
 	Purevpn               = "purevpn"
+	SlickVPN              = "slickvpn"
 	Surfshark             = "surfshark"
 	Torguard              = "torguard"
 	VPNUnlimited          = "vpn unlimited"
@@ -44,6 +45,7 @@ func All() []string {
 		Privatevpn,
 		Protonvpn,
 		Purevpn,
+		SlickVPN,
 		Surfshark,
 		Torguard,
 		VPNUnlimited,