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/bfs.go

@@ -0,0 +1,43 @@
+package html
+
+import (
+	"container/list"
+	"fmt"
+
+	"golang.org/x/net/html"
+)
+
+// BFS returns the node matching the match function and nil
+// if no node is found.
+func BFS(rootNode *html.Node, match MatchFunc) (node *html.Node) {
+	visited := make(map[*html.Node]struct{})
+	queue := list.New()
+	_ = queue.PushBack(rootNode)
+
+	for queue.Len() > 0 {
+		listElement := queue.Front()
+		node, ok := queue.Remove(listElement).(*html.Node)
+		if !ok {
+			panic(fmt.Sprintf("linked list has bad type %T", listElement.Value))
+		}
+
+		if node == nil {
+			continue
+		}
+
+		if _, ok := visited[node]; ok {
+			continue
+		}
+		visited[node] = struct{}{}
+
+		if match(node) {
+			return node
+		}
+
+		for child := node.FirstChild; child != nil; child = child.NextSibling {
+			_ = queue.PushBack(child)
+		}
+	}
+
+	return nil
+}