examples : generate JSON according to schema (#1887)

* examples : add JSON schema grammars

* complete JSON grammar

* ensure primitive types can be used as root of schema

* support integer type and adjust usage text

grammars/json.gbnf

@@ -1,29 +1,25 @@
-# Grammar for subset of JSON - doesn't support full string or number syntax
-
-root  ::= object
-value ::= object | array | string | number | boolean | "null"
+root   ::= object
+value  ::= object | array | string | number | ("true" | "false" | "null") ws
 
 object ::=
   "{" ws (
             string ":" ws value
     ("," ws string ":" ws value)*
-  )? "}"
+  )? "}" ws
 
 array  ::=
   "[" ws (
             value
     ("," ws value)*
-  )? "]"
+  )? "]" ws
 
-string  ::=
+string ::=
   "\"" (
     [^"\\] |
     "\\" (["\\/bfnrt] | "u" [0-9a-fA-F] [0-9a-fA-F] [0-9a-fA-F] [0-9a-fA-F]) # escapes
   )* "\"" ws
 
-# Only plain integers currently
-number  ::= "-"? [0-9]+ ws
-boolean ::= ("true" | "false") ws
+number ::= ("-"? ([0-9] | [1-9] [0-9]*)) ("." [0-9]+)? ([eE] [-+]? [0-9]+)? ws
 
 # Optional space: by convention, applied in this grammar after literal chars when allowed
 ws ::= ([ \t\n] ws)?