AST patterns for node equality

Often it's required to compare different parts of AST on equality to provide various inspections. For this, we need to provide new constructors for `PatternAst` and be able to compare patterns. For example, it would be desirable to specify a pattern for the expression:

```haskell
a && a
```

Which works on the following cases:

```haskell
True && True
x && x
not y && not y
foo bar Nothing baz && foo bar Nothing baz
```

Not sure, how to implement this, but a few ideas:

- [ ] Introduce the `PatternAstLabel Text` constructor where you can specify a tag for expression. So the above expression can be written as a pattern `op (label "a") andOp (label "a")`.
- [ ] Change the implementation of `hieMatchPatternAst` in such a way, that it works with `Map Text (HieAST TypeIndex)`. The idea is that when trying to match `PatternAstLabel` there could be 2 cases:
1. Such a label is not in `Map` yet: insert it into `Map`.
2. Such a label is already in `Map`: `lookup` in `Map` and compare current with what is there.
- [x] We need to figure out how to compare two `Node`s of `HieAST`. We can't use `==` because it will compare source positions, but we are not interested in them.

This is just an initial idea, but I think it will allow us to specify more custom rules and capture more patterns.

**UPDATE:** HIE AST comparison was implemented in #269

Start by reading the PatternAst design and the hieMatchPatternAst implementation described in the issue, then inspect the HIE AST comparison work referenced in #269. The intended result is a way to label a pattern and compare repeated AST nodes while ignoring source positions, but the remaining scope is not specified.

Written by the indexing model from the issue text.