sdk: add TTYSettings

[bjia56]

Intended to be used by btop plugin to add the path where btop is installed to the interactive terminal.

[bjia56]

Another possible implementation is to expose only one function, getTerminalSettings, and have that return a list (similar to the existing getSettings), where some list items may be labeled as "PATH"

[koush]

should return something like

getTerminalSettings(): Promise<{
  paths?: string[];
}>;

[koush]

Should maybe all be called TTYSettings per the existing TTY interface?