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/models/markdown.go

@@ -123,6 +123,8 @@ func getMarkdownHeaders(vpnProvider string) (headers []string) {
 		return []string{countryHeader, regionHeader, cityHeader, hostnameHeader, freeHeader}
 	case providers.Purevpn:
 		return []string{countryHeader, regionHeader, cityHeader, hostnameHeader, tcpHeader, udpHeader}
+	case providers.SlickVPN:
+		return []string{regionHeader, countryHeader, cityHeader, hostnameHeader}
 	case providers.Surfshark:
 		return []string{regionHeader, countryHeader, cityHeader, hostnameHeader, multiHopHeader, tcpHeader, udpHeader}
 	case providers.Torguard: