Support when for contextual keybindings

[denisidoro]

Is your feature request related to a problem? Please describe.
I'm not able to set keybindings contextually. For example, I would like to set <leader> + m + p as an action to preview content, and that depends on the programming language. At a higher level, I'm trying to replicate Spacemacs's major modes.

Using when seems to be a no-op. In the example below, both commands are executed regardless of the current editorLangId.

Describe the solution you'd like
To support when in elements inside commands, as in:

{"before":   ["<leader>", "m", "p"], 
 "after":    [], 
 "commands": [{"command": "markdown.showPreviewToSide", "args": [], "when": "editorLangId == 'markdown'"},
              {"command": "html.previewPage", "args": [], "when": "editorLangId == 'html'"}]}

Alternatively, when could be supported in the command root:

{"before":   ["<leader>", "m", "p"], 
 "after":    [], 
 "when":     "editorLangId == 'markdown'",
 "commands": [{"command": "markdown.showPreviewToSide", "args": []}]}

Describe alternatives you've considered
Set shorcuts in keybindings.json. I wouldn't be able to differ between vim modes, though. And I guess it would make my <leader> inconsistent.

[J-Fields]

Closing as duplicate of #2030