Always treat brace at command start as compound statement

For backwards compatibility, fish does not treat "{echo,hello}" as a compound
statement but as brace expansion (effectively "echo hello").  We interpret
"{X...}" as compound statement only if X is whitespace or ';' (which is an
interesting solution).

A brace expansion at the very start of a command 
is usually pointless (space separation is shorter).
The exception are cases where the command name and the first few arguments
share a suffix.

	$ {,1,2,3,4}echo
	1echo 2echo 3echo 4echo

Not sure if anyone uses anything like that.  Perhaps we want to trade
compatibility for simplicity. I don't have a strong opinion on this.

Always parse the opening brace as first character of a command token as
compound statement.
Brace expansion can still be used with a trick like: «''{echo,foo}»

Closes #11477

doc_src/language.rst

@@ -917,6 +917,12 @@ If there is nothing between a brace and a comma or two commas, it's interpreted
 
 To use a "," as an element, :ref:`quote <quotes>` or :ref:`escape <escapes>` it.
 
+The very first character of a command token is never interpreted as expanding brace, because it's the beginning of a :ref:`compound statement <cmd-begin>`::
+
+    > {echo hello, && echo world}
+    hello,
+    world
+
 .. _cartesian-product:
 
 Combining lists