Better remap-lang error reporting

[JeanMertz]

We should do another pass over the remap-lang implementation, and improve its error messages.

There are two areas we should improve:

1. The parser errors generated by the pest library are based on the rules we define in the grammar. These rules are sometimes named reasonable for internal use, but aren't as useful in error messages. We can leverage Pest's error manipulation API to improve this situation.

2. Parser errors show line and column numbers with a visual representation of the code that failed to compile. However, our custom errors do not, which makes them less easy to parse. We don't need to get too fancy here, but it would be nice if we could make sure the error messages are as clear as they can be (by providing whatever relevant context we can in the messages themselves), and perhaps add the line number where the error occurred (this is information we have).

[JeanMertz]

ref: #5876 (comment)

[JeanMertz]

ref: #5911 (comment)

We should improve this error (and others) to actually print the expression and provide a solution.

[JeanMertz]

Leaving this for reference, we should try to use the codespan crate to provide useful error messages when a program fails to compile.

For an example, see: https://raw.githubusercontent.com/brendanzab/codespan/132f4db0f568ae160b1a47510d1ef21d0223d6f3/codespan-reporting/assets/readme_preview.svg

We could also look into alternative crates, but they seem less feature-full or unmaintained.