Add 'trailing' option to 'editor.renderWhitespace'

[stnrepin]

This PR fixes #43127

Add a new option ('trailing') to editor.renderWhitespace that renders only trailing whitespace (including lines with only whitespace).

[ghost]

All CLA requirements met.

[alexdima]

Thank you!

[noinkling]

Any chance we could get a way to combine both the selection and trailing options? Maybe selection should be its own setting?

[stnrepin]

I also think the selection option doesn't correspond to other options in editor.renderWhitespace: it works like all but only for selected text, therefore playing a slightly different role. Moreover, as you said, extracting selection to its own setting will increase customization capabilities (will allow combine options). So, I like your idea.

My little investigation showed that realization shouldn't be hard, so I can create the issue (where we can discuss the details) and implement this feature if @alexdima finds it acceptable.

[krid]

There's now a relevant bug: #106697
Go forth and upvote it.