Automatically install extension when it gets activated by the lsp

[itepastra]

when the client lets the server autocomplete one of the hx-ext="{extension}" and the header is specified in the current tree, add the script source to it. ex. Add <script src="https://unpkg.com/htmx.org/dist/ext/event-header.js"></script> to the header when the extension is completed using the lsp:

<button hx-ext="event-header">
   Click Me!
</button>