Improve environment selection UX

[x1unix]

Problem

Since the first release, the playground had the functionality to change how and where Go program runs.
The problem is that option is placed in a counterintuitive place, so regular users don't even know this feature exists.

I tried to address this feature before and to place an additional status bar button that displays what environment is currently used and allows switching between Go server and WebAssembly, but apparently, it's still "invisible" for users.

Solution

I plan to introduce a simple dropdown to select Go environment in the same way as the original Go playground does.
This will also introduce custom Go versions support and allows extending a list of available execution options.

Mobile support

Fluent UI Dropdown element supports adaptive design and automatically switches to alternative UI on small screens, which is awesome.