Allow variable refs as object keys {$key:value}

Users are often surprised by the requirement to parenthesize any
non-trivial object key expressions in object constructors.  E.g.,
{"a"+"b":1}.  This commit adds one more kind of key expression besides
literals and idents: variable references.

A common use case for this is jq programs as JSON templates to fill in
with variables computed from inputs or passed in on the command-line.
E.g., {some_key:$value}.  Now users can also use, e.g., {$key:$value}.

This and the restrictions on key and value expressions in object
constructors are now clarified a bit in the documentation.

src/parser.y

@@ -928,6 +928,10 @@ IDENT ':' ExpD {
   $$ = gen_dictpair($1, BLOCK(gen_op_simple(POP), gen_op_simple(DUP2),
                               gen_op_simple(DUP2), gen_op_simple(INDEX)));
   }
+| '$' IDENT ':' ExpD {
+  $$ = gen_dictpair(gen_location(@$, locations, gen_op_unbound(LOADV, jv_string_value($2))),
+                    $4);
+  }
 | '$' IDENT {
   $$ = gen_dictpair(gen_const($2),
                     gen_location(@$, locations, gen_op_unbound(LOADV, jv_string_value($2))));