Record/keep extension source for later reuse (`quarto update extension ...`)

Currently, when installation an extension from GitHub, if the extension name does not match the repository name, it's not possible to use quarto update extension ....

The majority of extensions uses quarto-<extension name> as a repository name instead of <extension name>.
For example:

1. Install extensions:

   quarto add mcanouil/quarto-iconify
   quarto add quarto-ext/typst-templates/poster
   

2. Check the installed extensions

   quarto list extensions
   

   Id                         Version    Contributes        
   quarto-ext/poster          1.0.0      formats            
   mcanouil/iconify           2.1.0      shortcodes         
   

3. Update the extensions

   quarto update extension quarto-ext/poster
   

   Extension not found in local or remote sources
   

This means that it's not possible to update the extension quarto-ext/poster because the repository name (directory) is typst-templates/poster and not poster.
Same goes for the extension mcanouil/iconify.

This could be solved by recording/keeping the repository name (source) somewhere.
It could be in _extension.yml under source key.

Keeping the source would allow better extension management and easier updates, e.g., quarto update extension all.

Start with the implementation of the quarto add extension, quarto list extensions, and quarto update extension commands, then inspect the proposed _extension.yml source metadata. Done means extensions installed from repositories with different names can retain their source and be found by quarto update extension, including all.