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.

tests/checks/braces.fish

@@ -187,10 +187,26 @@ PATH= "{"
 # CHECKERR: PATH= "{"
 # CHECKERR:       ^~^
 
+$fish -c '{ -h'
+# CHECKERR: fish: '{': missing man page
+# CHECKERR: Documentation may not be installed.
+# CHECKERR: `help '{'` will show an online version
+
+PATH= "{" -h
+# CHECKERR: fish: Unknown command: '{'
+# CHECKERR: {{.*}}/braces.fish (line {{\d+}}):
+# CHECKERR: PATH= "{" -h
+# CHECKERR:       ^~^
+
 $fish -c 'builtin {'
 # CHECKERR: fish: Expected end of the statement, but found a '{'
 # CHECKERR: builtin {
-# CHECKERR:      ^
+# CHECKERR:         ^
+
+$fish -c 'builtin "{"'
+# CHECKERR: fish: Unknown builtin '"{"'
+# CHECKERR: builtin "{"
+# CHECKERR: ^~~~~~~~~~^
 
 $fish -c 'command {'
 # CHECKERR: fish: Expected end of the statement, but found a '{'

tests/checks/indent.fish

@@ -482,6 +482,9 @@ level 2 } }
     # CHECK: {{^        level 2$}}
     # CHECK: {{^    \}$}}
     # CHECK: {{^\}$}}
+
+echo '{  -h'
+# CHECK: { -h
 } | $fish_indent
 
 echo 'multiline-\\