Feature: format selection

I'm new to ReScript and I've been trying the VS Code extension with Format on Save enabled. However, I'd like to manually control formatting with the Format Selection command (more on my reasons here). But the extension's formatter only seems to support whole file formatting. Neither Format Selection, nor setting the Format on Save Mode to "modifications", seem to do anything.

This would be an important feature for me since I don't think I can carry on with Format on Save enabled (I don't like some of the formatter's behaviour) but I'd like to stick to standard formatting as much as possible so it would help a lot with that.

I don't know if this is an issue with the extension or with the formatter itself being unable to handle partial content, but I thought I'd create the issue for the component where I experience the problem.

Start by tracing the VS Code extension's formatter entry points for Format Selection and the Format on Save Mode setting. Determine whether partial-content formatting is handled by the extension or the formatter; done means both selection formatting and modification-based formatting produce the expected result.