Support help argument in "{ -h"

Unlike other builtins, "{" is a separate token, not a keyword-string
token.

Allow the left brace token as command string; produce it when parsing
"{ -h"/"{ --help" (and nowhere else).  By using a decorated statement,
we reuse logic for redirections etc.

Other syntax elements like "and" are in the builtin list, which
- adds highlighting logic
- adds it to "builtin --names"
- makes it runnable as builtin
  (e.g. "builtin '{'" would hypothetically print the man page)

These don't seem very important (highlighting for '{' needs to match
'}' anyway).

Additionally, making it a real builtin would mean that we'd need to
deactivate a few places that unescape "{" to BRACE_BEGIN.

Let's not add it to the built in list. Instead, simply synthesize
builtin_generic in the right spot.

I'm assuming we want "{ -h" to print help, but '"{" -h' to run an
external command, since the latter is historical behavior.  This works
naturally with the above fake builtin approach which never tries to
unescape the left brace.

share/functions/__fish_print_help.fish

@@ -8,6 +8,8 @@ function __fish_print_help --description "Print help message for the specified f
             set item true
         case '['
             set item test
+        case '{'
+            set item begin
     end
 
     # Do nothing if the file does not exist