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/updater/html/errors.go

@@ -0,0 +1,27 @@
+package html
+
+import (
+	"bytes"
+	"fmt"
+
+	"golang.org/x/net/html"
+)
+
+func WrapError(sentinelError error, node *html.Node) error {
+	return fmt.Errorf("%w: in HTML code: %s",
+		sentinelError, mustRenderHTML(node))
+}
+
+func WrapWarning(warning string, node *html.Node) string {
+	return fmt.Sprintf("%s: in HTML code: %s",
+		warning, mustRenderHTML(node))
+}
+
+func mustRenderHTML(node *html.Node) (rendered string) {
+	stringBuffer := bytes.NewBufferString("")
+	err := html.Render(stringBuffer, node)
+	if err != nil {
+		panic(err)
+	}
+	return stringBuffer.String()
+}