enhance: OpenVSX message

[daiyam]

Add a message on top of the Extensions panel to indicate that we are using OpenVSX

[SnorlaxMonster]

The search bar on the Extensions panel currently says "Search Extensions in Marketplace". I think swapping that to just saying "Search Extensions in Open VSX" would be a good idea (since "Marketplace" here is short for "Visual Studio Marketplace").

A dedicated message might be nice too, but I think clarifying that in the search panel would be a good start at the very least.

[SnorlaxMonster]

Similarly, in the right panel of the extension view, the link to Open VSX entry for the extension is currently labelled "Marketplace". Ideally that would also be replaced with "Open VSX" to clarify where that link leads.