Warn for pure unused functions

[GearsDatapacks]

Closes #4477

[lpil]

Fantastic! Thank you. I've left some notes inline.

Purity hasn't been included in the package interface, which I think is a good idea for now.

It could be even more useful if the compiler knows that functions such as list.reverse and dict.get are pure. Do we want to have a hard-coded list of pure stdlib functions?

[GearsDatapacks]

I think having a list of pure stdlib functions is good, especially for things like dict.insert which in a non-functional language would not return anything and could definitely be a source of confusion.

[GearsDatapacks]

I've now added the is_pure_standard_library_function function which matches against all the current standard library functions except:

• gleam/io.* - those are inherently impure
• The deprecated gleam/dynamic modules - they will soon be removed and be redundant

[GearsDatapacks]

Based on our discussion on Discord, I've reverted some of my changes to the purity tracking system to make it simple but effective enough for the current use case.
I've also added a comment to the Purity enum explaining the limitations of this system and how it can be improved in future.

[lpil]

Fantastic! Really liking this. I've left a few notes inline about some comments, just to make sure we understand this in future as it's conceptually quite complex.

[lpil]

Really fantastic documentation there! I love it. Thank you!