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>

.github/labels.yml

@@ -64,6 +64,9 @@
 - name: ":cloud: PureVPN"
   color: "cfe8d4"
   description: ""
+- name: ":cloud: SlickVPN"
+  color: "cfe8d4"
+  description: ""
 - name: ":cloud: Surfshark"
   color: "cfe8d4"
   description: ""