Add pipelines and pipeline elements

[kubouch]

Currently, pipelines and pipeline elements are missing. In Nushell the hierarchy is

block -> pipeline -> pipeline element -> expression

while we have only

block -> AST node

[sholderbach]

Is this full hierarchy useful in IR land? I guess it might make sense to have a statement/expression level, but for transformations we may want to do are the pipeline/pipeline elemen hierarchies still as useful. (e.g. if we want to lift a | get bla to a direct cell path access etc.

[kubouch]

Lifting | get bla to a direct cell path access would require get to be a parser keyword (or done later via some IR optimization?). I haven't looked closely to the IR yet, so I can't tell if it would be better with or without pipelines, but we'll need some way to track command inputs/outputs. We'll also need it at the AST level to parse and typecheck it properly.