command $foo broken highlighting

[mqudsi]

fish 3.0 added support to use command $foo to execute the command whose name/path is stored in $foo, but the syntax highlighting code was not updated to take into account that a variable is now can be a valid execution target, and as a result $foo is always highlighted in red (since there is no executable literally named $foo).

e.g.

[floam]

Same thing with builtin $foo after 038fea1.

[mqudsi]

and plain, old $foo:

[ridiculousfish]

Yeah I've been chewing over how to fix this for a while

[zx8]

Related: #5571