Add syntax highlighting for directives specified using the colon fence

[jmarknixon]

Is your feature request related to a problem? Please describe. I love how the colon fence plugin renders directives that use it (for example, the note admonition) in previews. I'd like to see the plugin also perform syntax highlighting in the source.

Describe the solution you'd like When using the colon fence with a directive, I'd like to see the same highlighting I see when I use the backtick or tilde fence. See the attached screenshot.

Describe alternatives you've considered N/A

Additional context

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[atrnh]

~@jmarknixon It looks like highlighting for colon fences was introduced in https://github.com/executablebooks/myst-vs-code/commit/81db9724125928743381a84088343430417388f7. So, according to the commit message, I think this feature should work if we add 'colon_fence' to the myst.preview.extensions setting.~

Nope, I'm totally wrong—that only enables colon fences rendering in the preview. Seems obvious now. Whoopsie 😅

[LoicGrobol]

Given the current structure of the extension ,this should probably go to https://github.com/chrisjsewell/vscode-myst-syntax