Add Markdown Underline Style to Get Texts Underlined (Maybe Not In Markdown)

[GreenYun]

Some language syntax extensions, like rust-analyzer, may use "markup.underline" to distinguish terms. Just add the following code copied from the default theme:

		{
			"scope": "markup.underline",
			"settings": {
				"fontStyle": "underline"
			}
		},

[GreenYun]

Duplicated #139 due to GitHub 500 Server error. Closed.