Add support for vim

[ianthomas23]

Add support for vim editor. This is the first wasm command that uses the whole terminal and performs complex input-output whilst it is running.

Here are some screenshots showing coloured syntax highlighting and spell checking (using set: spell):

Key implementation changes are handling of a few more termios settings, and monkey patching a new poll function into loaded wasm modules so that they can poll to see if input is ready to read.

Limitations:

• It doesn't include docs and tutorial which is by design to reduce the size of the compiled package. But there are still links to the docs which will error out if accessed.
• Does not respond to resizing the terminal whilst vim is running.
• It is not possible yet to use a bespoke vimrc file.