[ST4] Add new context keys.

[UltraInstinct05]

This PR adds context keys that have been added in various builds of ST4 starting from 4050. The following table summarizes them

Key	Operand	Operators	Match All	Description	Added in
"overlay_has_focus"	boolean	equality	no	Whether the overlay currently open, has focus.	4082
"overlay_name"	string	equality	no	The name of the overlay. Valid values are "goto" & "command_palette"	4082
"group_has_multiselect"	boolean	equality	no	Whether the currently focused group has multi selected tabs.	4050
"group_has_transient_sheet"	boolean	equality	no	Whether the currently focused group has a transient sheet.	4050
"has_snippet"	boolean	equality	no	Whether the immediate word before the cursor can be expanded to a snippet.	4050
"is_javadoc"	boolean	equality	yes		4050

As to the is_javadoc one, I have no idea how to trigger it & what it does. So I have marked the details for it as a TODO.

[keith-hall]

Fwiw, your summary for has_snippet is incorrect, but you got it right in the completion file - just writing about it here in case anyone else is confused by that.
Thanks for this btw :)

[UltraInstinct05]

Thanks. I'll edit & correct it.

[UltraInstinct05]

I had forgotten to add these context keys to the syntax file for appropriate highlighting. I have made a commit that does so.

[FichteFoll]

Thanks for the additions. Should perhaps also notify Will that these aren't in the ST4 docs either, if he's not aware of it already.