More error context when parsing haskell expression

[Carbon based Guibou]

Right now, when parsing haskell expression, we just dump the TH Exp in the AST and are happy with it.

However, if this code is invalid, it will fail in a later stage with an imprecise error. For example:

/home/guillaume/PyFError.hs:6:17: error:
    Variable not in scope: name2
  |
6 |   putStrLn [fmt|Bonjour {name2:o}|]
  |                 ^^^^^^^^^^^^^^^^^^^

We cannot do much, but we can at least evaluate the AST to check for missing variables, using https://hackage.haskell.org/package/template-haskell-2.18.0.0/docs/Language-Haskell-TH.html#v:lookupValueName to check for the existance of the name. I would like an error message such as:

/home/guillaume/PyFError.hs:6:40-45: error:
    Variable not in scope: name2
  |
6 |   putStrLn [fmt|Bonjour {name2:o}|]
  |                          ^^^^^

This is perfectly doable right now, either:

• the evalExpr function may do the check and reports the errors
• evalExpr should return the evaluated expression as TH code (i.e. Exp), as well as the original AST (HsExpr GhcPs). Later, we can walk this AST to check for errors.

[Carbon based Guibou]

Working a bit on this topic in #106 I progressively realize that:

• Instead of failing immediately, we can gather all errors and display all of them at the end, it will give a better experience
• I'm wondering if it may be possible to use GHC to type check the interpolated expression and based on that, do more check and do the formatter dispatch. I'll evaluate that.