Implement pipeline

[WindSoilder]

Closes: #37

This pr introduces a new AstNode::Pipeline variant. When parsing block, it may run into pipeline_or_expression_or_assignment which may return Pipeline/Expression/Assignment. And it enables let and mut using pipelines, so the following is possible:

let x = 3 | 4
mut y = 5 | 6
$y = 7 | 8 | 9

Changes in detail

• In math_expression, change rhs from expression to pipeline_or_expression, so $y = 7 | 8 | 9 is available, it also returns MathExpressionNode, so it can be used easier in pipeline_or_expression_or_assignment.
• In let_statement, mut_statement, change initializer to pipeline_or_expression, so let x = 7 | 8, mut y = 5 | 6 is available
• The core logic lays inside pipeline_or_expression, in detail, if it get the first expression, and it failed to read a pipe, then it returns an Expression node, or else it will consume pipes and return an Pipeline node.

[kubouch]

I think it's a good start, just a few small things but we can merge it otherwise.

I actually started working on this but never finished it, it's on my pipeline branch of my fork: https://github.com/kubouch/new-nu-parser/tree/pipeline. There I went a bit further and started implementing also the redirections. Also I added there a Sentence enum that contains either a pipeline, or a statement, not sure if that's the best way. If you happen to work on it, you can take my code, but let me know, otherwise I might attempt to finish it at some point :-).

[WindSoilder]

Thanks! I'll look what you have done before :-)
Also I update the github ci to use ubuntu-22.04, because ubuntu-20.04 is going to be removed.

[kubouch]

OK, looks good. Can you put the CI change to a separate PR? Could be even 24.04.