Unify completion pos usage, fix presentation compiler crash in interpolation

[rochala]

Fixes crash for added tests in CompletionInterpolatorSuite along with unification and better naming for completion pos, source pos, and span usage.

[rochala]

I also removed 2 files: Custom completions and its Tests. They were added to the compiler for ammonite, but after all they were also never used. They failed the tests, and the reason behind it that the file hasn't been updated for 3 years, and getting it up to date is too much costs / not worth.

[rochala]

They were added to the compiler for ammonite, but after all they were also never used. They failed the tests, and the reason behind it that the file hasn't been updated for 3 years, and getting it up to date is too much costs / not worth.

We also have AmmoniteIvyCompletions now, which should do the same thing.

This is actually a bit different. They planned to use this in repl completions.