Add string.commands
Add stubs to string.c
Add string files to CMakeLists.txt
Add ws_builtin_cmd_strcat implementation
Add ws_builtin_cmd_substr implementation
Add static function get_strings_from_union
Add implemenation of ws_builtin_cmd_strcmp
Add error output to main.c
Merge pull request #364 from xVin/Add-error_output-in-main

Add error output to main.c
Merge pull request #362 from xVin/Add-string_commands

[WIP] Add string commands