This adds initial support for statements with prefixed variable assignments.
Statments like this are supported:
a=1 b=$a echo $b # outputs 1
Just like in other shells, the left-hand side of each assignment must
be a valid variable identifier (no quoting/escaping). Array indexing
(PATH[1]=/bin ls $PATH) is *not* yet supported, but can be added fairly
easily.
The right hand side may be any valid string token, like a command
substitution, or a brace expansion.
Since `a=* foo` is equivalent to `begin set -lx a *; foo; end`,
the assignment, like `set`, uses nullglob behavior, e.g. below command
can safely be used to check if a directory is empty.
x=/nothing/{,.}* test (count $x) -eq 0
Generic file completion is done after the equal sign, so for example
pressing tab after something like `HOME=/` completes files in the
root directory
Subcommand completion works, so something like
`GIT_DIR=repo.git and command git ` correctly calls git completions
(but the git completion does not use the variable as of now).
The variable assignment is highlighted like an argument.
Closes#6048
Arrow keys are often not conveniently located on keyboards, so the use of arrow keys for common keyboard shortcuts can be a turn-off for some.
I found that fish supports alternate keybindings for these cases but I didn't seem them documented in these places where the arrow keys versions are highlighted.
This adds a reference to one specific FAQ, so it adds a label for that
one question. It does not add the rest, because they currently aren't
linked. If you add a reference to an FAQ, you should add the label as
well.
[ci skip]
