Add support for 'Current Document' option for background execution of analyzers

Visual Studio supports these 4 values for background analysis scope option: `Current Document`, `Open Documents`, `Full Solution`, `Disabled or None`

VSCode C# extension does not support the `Current Document` option. Note that this value was added and made the default for background execution of analyzers as running all analyzers on all open documents was performance intensive on larger solutions with large number of files open in the session, and did not seem to be of any significant value add to the user.

This issue tracks adding `Current Document` support in the C# extension and make it the default for analyzer execution in background. I believe @dibarbet mentioned this would need some LSP work to be able to know the active document context in the extension.