Lex $foo as a single token, instead of using '$' IDENT

Previously, bindings were parsed as the combination of two tokens:
  '$' IDENT

This meant that using a keyword as variable name (e.g. $then, $label)
did not work.
Attempts were made to allow $keyword to work by adding some '$' Keyword
rules in the parser, but this did not allow $keyword in all places:

  jq --arg label foo -n '$label'  # ok

  jq -n '"foo" as $label | .'     # error

Treating $foo as a single token is much simpler, in my opinion.

This patch also changes how LOC is lexed: "$__loc__" instead of as
"__loc__" that gets combined with '$' in the parser.

This patch also disallows having spaces after '$' when recalling a
variable  `$ foo'  since that was probably just an unintentional side
effect of the implementation, and it was not documented.

Fixes #2675

src/lexer.l

@@ -60,7 +60,7 @@ struct lexer_param;
 "catch" { return CATCH; }
 "label" { return LABEL; }
 "break" { return BREAK; }
-"__loc__" { return LOC; }
+"$__loc__" { return LOC; }
 "|=" { return SETPIPE; }
 "+=" { return SETPLUS; }
 "-=" { return SETMINUS; }
@@ -122,6 +122,7 @@ struct lexer_param;
 
 ([a-zA-Z_][a-zA-Z_0-9]*::)*[a-zA-Z_][a-zA-Z_0-9]*  { yylval->literal = jv_string(yytext); return IDENT;}
 \.[a-zA-Z_][a-zA-Z_0-9]*  { yylval->literal = jv_string(yytext+1); return FIELD;}
+\$([a-zA-Z_][a-zA-Z_0-9]*::)*[a-zA-Z_][a-zA-Z_0-9]*  { yylval->literal = jv_string(yytext+1); return BINDING;}
 
 [ \n\t]+  {}