Fix input prompt quoting issue

[jonsequitur]

This fixes a couple of issues:

• Type hints weren't correctly inferred when input prompts weren't in quotes.
• Multiple #!set commands with @input tokens in a single submission were all getting the same value.

There's also some work here refactoring toward #3323 but the design is just a sketch and will be refactored toward a more ergonomic syntax. (See the discussion here.