LibVT: Implement new ANSI escape sequence parser

This commit replaces the former, hand-written parser with a new one that
can be generated automatically according to a state change diagram.

The new `EscapeSequenceParser` class provides a more ergonomic interface
to dealing with escape sequences. This interface has been inspired by
Alacritty's [vte library](https://github.com/alacritty/vte/).

I tried to avoid changing the application logic inside the `Terminal`
class. While this code has not been thoroughly tested, I can't find
regressions in the basic command line utilities or `vttest`.

`Terminal` now displays nicer debug messages when it encounters an
unknown escape sequence. Defensive programming and bounds checks have
been added where we access parameters, and as a result, we can now
endure 4-5 seconds of `cat /dev/urandom`. :D

We generate EscapeSequenceStateMachine.h when building the in-kernel
LibVT, and we assume that the file is already in place when the userland
library is being built. This will probably cause problems later on, but
I can't find a way to do it nicely.

Userland/Libraries/LibVT/CMakeLists.txt

@@ -1,7 +1,11 @@
+# FIXME: this assumes that EscapeSequenceStateMachine.h has been
+# already generated when the kernel was built. This will probably
+# mess builds up later on.
 set(SOURCES
     Line.cpp
     Terminal.cpp
     TerminalWidget.cpp
+    EscapeSequenceParser.cpp
 )
 
 serenity_lib(LibVT vt)