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

src/fish_tests.cpp

@@ -4626,6 +4626,7 @@ static void test_highlighting() {
     });
 
     highlight_tests.push_back({
+        {L"HOME=.", highlight_role_t::param},
         {L"false", highlight_role_t::command},
         {L"|&", highlight_role_t::error},
         {L"true", highlight_role_t::command},