Show type of thing the cursor is currently on at the REPL

[byorgey]

How about showing the type of function the cursor is on?

Originally posted by @xsebek in #1955 (comment)

Concretely, I think the idea should be to show the type (in the UR corner of the REPL panel) of the most deeply nested part of the input AST that the cursor is on or after. For example (the cursor position is indicated by |):

• build {give | } would display give : Actor -> Text -> Cmd Unit
• foo ((2 + 3) | + 7) would display 2 + 3 : Int

[byorgey]

I played with this a bit, and the biggest hurdle is the fact that right now, either a term typechecks completely or not at all. If one part of a term does not typecheck, it is not possible to get type information about the other parts that did. Ideally this should come after addressing #1483 - we can return a term annotated with as much type information as possible, and type errors marked as well, which will allow us to still extract type information about some parts of the term.