Better parse error messages for reserved words used incorrectly

[byorgey]

Trying to load a file containing

if : N
if = 0

results in

./bad.disco:1:1:
  |
1 | if : N
  | ^
unexpected 'i'
expecting "import", "using", or end of input

The error message does not say anything about the fact that if is a reserved word.

[byorgey]

The test case for #283 now suffers from this problem as well, now that REPL inputs are parsed as modules (once #304 is merged).

[byorgey]

Note that just typing if by itself already illustrates the issue:

Disco> if
1:1:
  |
1 | if
  | ^
unexpected 'i'
expecting "import", "operator", "using", ':', or end of input

[byorgey]

The problem seems to arise whenever there are multiple options: whichever option involves parsing an expression fails because if is not an identifier, so the other options are used instead. I guess what we want to do is something like specify which error message(s) take priority if all the possible alternatives fail?

[byorgey]

Might require researching a bit about megaparsec's error reporting mechanisms.

[byorgey]

Something from Willis & Wu "Design Patterns for Parser Combinators" might help? https://dl.acm.org/doi/10.1145/3471874.3472984