Added a few builtin functions to the shell to make navigating a bit
easier in the terminal.
`pushd` allows a user to "push" the current directory to the directory
stack, and then `cd` to the new directory.
`popd` allows the used to take the directory on the top of the stack
off before `cd`'ing to it.
`dirs` gives the state of the current directory stack.
This is only a partial implementation of the `bash` version
(gnu.org/software/bash/manual/html_node/Directory-Stack-Builtins.html)
, and doesn't include any of the +N or -N commands as of yet.
This was a workaround to be able to build on case-insensitive file
systems where it might get confused about <string.h> vs <String.h>.
Let's just not support building that way, so String.h can have an
objectively nicer name. :^)
When resizing the terminal, we now clear the entire current line and reset
the shell's LineEditor input state. This makes it look and feel kinda the
same as xterm.
Fixes#286.
