Prompt callbacks

libdash, by default, uses dash expansion prints the `PS1` and `PS2` in the dash prompt. This behavior is wrong (and possibly dangerous, depending on the use case).

We should make the prompt functions monkeypatchable, allowing users of the library to circumvent dash internals (beyond the alias table) entirely. In an ideal world, we could even compile just the few files necessary.

Start by tracing the prompt functions involved in dash expansion and how PS1 and PS2 are printed. Determine the callback boundary needed to bypass those internals, then verify that users can replace the prompt behavior and that only the necessary files can be compiled.

Written by the indexing model from the issue text.