Support FOO=bar syntax for passing variables to individual commands

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

sphinx_doc_src/cmds/set.rst

@@ -119,6 +119,12 @@ Examples
         echo "Python is at $python_path"
     end
 
+    # Like other shells, fish 3.1 supports this syntax for passing a variable to just one command:
+    # Run fish with a temporary home directory.
+    HOME=(mktemp -d) fish
+    # Which is essentially the same as:
+    begin; set -lx HOME (mktemp -d); fish; end
+
 Notes
 -----