Add additional jsonpath string methods

Add additional jsonpath string methods

Add the following jsonpath methods:

*   l/r/btrim()
*   lower(), upper()
*   initcap()
*   replace()
*   split_part()

Each simply dispatches to the standard string processing functions.
These depend on the locale, but since it's set at `initdb`, they can be
considered immutable and therefore allowed in any jsonpath expression.

Author: Florents Tselai <florents.tselai@gmail.com>
Co-authored-by: David E. Wheeler <david@justatheory.com>
Reviewed-by: Chao Li <li.evan.chao@gmail.com>
Reviewed-by: Andrew Dunstan <andrew@dunslane.net>
Discussion: https://postgr.es/m/CA+v5N40sJF39m0v7h=QN86zGp0CUf9F1WKasnZy9nNVj_VhCZQ@mail.gmail.com