Move rust keybindings to seperate ftplugins file.

[rohanorton]

In lsp-installer.lua the language servers are being setup by looping through an array of server names rather than the deprecated on_server_ready event. As a result, each server is setup at start-time. This means that the rust specific keybindings are set for all filetypes rather than for just rust specifically.

I moved the keymaps into their own rust specific ftplugin file to ensure that they are only set for rust files.