Add "Sort Selection" command

[bradlc]

This PR adds a new "Sort Selection" command to the extension. When activated this command will sort the selection(s) in the same way that prettier-plugin-tailwindcss sorts lists of CSS classes.

This useful when you want to sort a list of classes that prettier-plugin-tailwindcss does not handle, e.g.

const myVar = 'these classes here'

Sort.Selection.Demo.mov

The command is only available when:

• The editor has one or more active text selections
• There is an active Tailwind project for the current active file
• The Tailwind version for the active Tailwind project is at least 3.0.0
• The active file is not excluded (via tailwindCSS.files.exclude)

[Benbinbin]

really need "sort classes name" as the built-in feature the extension, maybe take a further step to make this feature as "sort on save" will be more convenient