[feature request] editor.wordHighlightBorderStyle et al.

[will-stone]

• VSCode Version: 1.21
• OS Version: macOS Sierra/HS

With the recent release, themers are able to add borders to highlighted words (thank-you for this). However, it'd be nice to be able change the style of the borders using something like:

"editor.wordHighlightBorderStyle": "solid" / "editor.wordHighlightBorder.style": "solid"

Then provide solid, dashed, dotted as options.

[kanlukasz]

It could be merge with this -> #46804

[rebornix]

Let's track in #19534