psql: add \create_function command

Hello hackers,

Currently a function definition must include its body inline. Because of
this, when storing function definitions in files, linters and syntax
highlighters for non-SQL languages (python, perl, tcl, etc) won't work. An
example can be seen on:

https://github.com/postgres/postgres/blob/5eafacd2797dc0b04a0bde25fbf26bf79903e7c2/src/pl/plpython/sql/plpython_test.sql#L15-L24

To solve the above issue, this patch adds a psql command to create a
function and obtain its body from another file. It is used as:

\create_function from ./data/max.py max(int,int) returns int LANGUAGE
plpython3u

Its design is similar to the `\copy` command, which is a frontend version
of the COPY statement.

This patch is at an initial stage but includes tests with plpython3u,
pltcl, plperl and tab completion.

Any feedback is welcomed.

Best regards,
Steve Chavez