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

tests/jq.test

@@ -1770,3 +1770,25 @@ false
 1
 1
 
+
+# Using a keyword as variable/label name
+
+123 as $label | $label
+null
+123
+
+[ label $if | range(10) | ., (select(. == 5) | break $if) ]
+null
+[0,1,2,3,4,5]
+
+reduce .[] as $then (4 as $else | $else; . as $elif | . + $then * $elif)
+[1,2,3]
+96
+
+1 as $foreach | 2 as $and | 3 as $or | { $foreach, $and, $or, a }
+{"a":4,"b":5}
+{"foreach":1,"and":2,"or":3,"a":4}
+
+[ foreach .[] as $try (1 as $catch | $catch - 1; . + $try; .) ]
+[10,9,8,7]
+[10,19,27,34]