Add json_typeof() and json_is_*() functions.

The attached patch adds four new SQL functions for the JSON type:
json_typeof(json) RETURNS text
json_is_object(json) RETURNS boolean
json_is_array(json) RETURNS boolean
json_is_scalar(json) RETURNS boolean

The motivating use-case for this patch is the ability to easily create a
domain type for what RFC 4627 calls "json text", where the top-level value
must be either an object or array. An example of this usage is:

CREATE DOMAIN json_document AS json CHECK (NOT json_is_scalar(VALUE));

An additional use-case arises when writing functions which can handle
arbitrary JSON values. This can be difficult when nested objects or arrays
are present or when the input may be either an array or an object. Many of
the built-in functions will raise an error when presented with an "invalid"
value, such as when giving an array to json_object_keys(). The
json_typeof() and json_is_*() functions should make it easier to call the
correct function in these cases, e.g.:

CASE json_typeof($1)
WHEN 'object' THEN json_object_keys($1)
WHEN 'array' THEN json_array_elements($1)
ELSE $1
END

These new functions operate by making a single call to json_lex() to get
the first token of the JSON value; this token uniquely determines the
value's type. (Thanks to Merlin Moncure for suggesting this approach.)

The patch also updates the "JSON Functions and Operators" section of the
docs to ensure that the words "value", "object", and "array" are used in a
consistent manner. "JSON object" and "JSON array" refer to parameters
which must be an object or an array or to results which are always an
object or an array. "JSON value" refers to parameters or results which may
be any kind of JSON.

Regards,
Andrew Tipton