PATCH: jsonpath string methods: lower, upper, initcap, l/r/btrim, replace, split_part

Hello hackers,

This patch is a follow-up and generalization to [0].

It adds the following jsonpath methods: lower, upper, initcap, l/r/btrim,
replace, split_part.

It makes jsonpath able to support expressions like these:

select jsonb_path_query('" hElLo WorlD "',
'$.btrim().lower().upper().lower().replace("hello","bye") starts with
"bye"');
select jsonb_path_query('"abc~(at)~def~@~ghi"', '$.split_part("~(at)~", 2)')

They, of course, forward their implementation to the internal
pg_proc-registered function.

As a first wip/poc I've picked the functions I typically need to clean up
JSON data.
I've also added a README.jsonpath with documentation on how to add a new
jsonpath method.
If I had this available when I started, it would have saved me some time.
So, I am leaving it here for the next hacker.

This patch is not particularly intrusive to existing code:
Afaict, the only struct I've touched is JsonPathParseItem , where I added {
JsonPathParseItem *arg0, *arg1; } method_args.
Up until now, most of the jsonpath methods that accept arguments rely on
left/right operands,
which works, but it could be more convenient for future more complex
methods.
I've also added the appropriate jspGetArgX(JsonPathItem *v, JsonPathItem
*a).

Open items
- What happens if the jsonpath standard adds a new method by the same name?
A.D. mentioned this in [0] with the proposal of having a prefix like pg_ or
initial-upper letter.
- Still using the default collation like the rest of the jsonpath code.
- documentation N/A yet
- I do realize that the process of adding a new method sketches an
imaginary.
CREATE JSONPATH FUNCTION. This has been on the back of my mind for some
time now,
but I can't say I have an action plan for this yet.

GitHub PR view if you prefer:
https://github.com/Florents-Tselai/postgres/pull/18

[0]
https://www.postgresql.org/message-id/flat/185BF814-9225-46DB-B1A1-6468CF2C8B63%40justatheory.com#1850a37a98198974cf543aefe225ba56

All the best,
Flo