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/provider/slickvpn/updater/helpers_test.go

@@ -0,0 +1,26 @@
+package updater
+
+import (
+	"os"
+	"strings"
+	"testing"
+
+	"github.com/stretchr/testify/require"
+	"golang.org/x/net/html"
+)
+
+func parseTestHTML(t *testing.T, htmlString string) *html.Node {
+	t.Helper()
+	rootNode, err := html.Parse(strings.NewReader(htmlString))
+	require.NoError(t, err)
+	return rootNode
+}
+
+func parseTestDataIndexHTML(t *testing.T) *html.Node {
+	t.Helper()
+
+	data, err := os.ReadFile("testdata/index.html")
+	require.NoError(t, err)
+
+	return parseTestHTML(t, string(data))
+}