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/match.go

@@ -0,0 +1,43 @@
+package html
+
+import (
+	"golang.org/x/net/html"
+)
+
+type MatchFunc func(node *html.Node) (match bool)
+
+func MatchID(id string) MatchFunc {
+	return func(node *html.Node) (match bool) {
+		if node == nil {
+			return false
+		}
+
+		return Attribute(node, "id") == id
+	}
+}
+
+func MatchData(data string) MatchFunc {
+	return func(node *html.Node) (match bool) {
+		return node != nil && node.Type == html.ElementNode && node.Data == data
+	}
+}
+
+func DirectChild(parent *html.Node,
+	matchFunc MatchFunc) (child *html.Node) {
+	for child := parent.FirstChild; child != nil; child = child.NextSibling {
+		if matchFunc(child) {
+			return child
+		}
+	}
+	return nil
+}
+
+func DirectChildren(parent *html.Node,
+	matchFunc MatchFunc) (children []*html.Node) {
+	for child := parent.FirstChild; child != nil; child = child.NextSibling {
+		if matchFunc(child) {
+			children = append(children, child)
+		}
+	}
+	return children
+}