add TextBuffer

[bingmatv]

There's FrameBuffer but no TextBuffer.

[bjorn3]

If you boot using BIOS, you can use VGA text mode to directly feed text to the GPU. Bowever in the case of UEFI, GOP (the UEFI replacement for the VGA and VESA device interfaces) only supports setting up framebuffers. Because of this the kernel has to turn text into pixels itself. For consistency the bootloader crate only provides a framebuffer interface, even on BIOS where it could technically provide a text mode interface.