Unify naming of Extensions Gallery vs. Extensions Marketplace

Description

Across the codebase, we sometimes refer to the VSCode Extensions Marketplace as an Extensions Gallery. Sometimes VSCode itself does this, but let's at least be consistent on our side.

Proposal

Let's stick with Extensions Marketplace as much as possible since that's the name of the feature we have in our documentation.

Investigation

TBD... What are the lines of code that reference Extensions Gallery?