slang: Add new package

slang: Add new package

From the S-Lang website:
S-Lang is a multi-platform programmer's library designed to allow a
developer to create robust multi-platform software. It provides
facilities required by interactive applications such as display/screen
management, keyboard input, keymaps, and so on. The most exciting
feature of the library is the slang interpreter that may be easily
embedded into a program to make it extensible. While the emphasis has
always been on the embedded nature of the interpreter, it may also be
used in a stand-alone fashion through the use of slsh, which is part of
the S-Lang distribution.

Adapted from the old packages feed:
https://git.openwrt.org/?p=openwrt/svn-archive/packages.git;a=tree;f=libs/slang2

Signed-off-by: Jeffery To <jeffery.to@gmail.com>