refactor: separate expression tree nodes into a separate tree from ast

[rodesai]

Description

This patch moves the nodes in our expression tree into a separate parse
tree. This is a first step towards moving the code that deals with
expressions into ksql-execution (see #3125),
so that the serialized execution plan nodes can refer to expressions (e.g. for a select).

Both AST and Expression node have a common parent Node class, which now
just contains a NodeLocation, and abstract hashcode/equals methods.

The Expression tree now has its own visitor interface, called
ExpressionVisitor. This patch also migrates ExpressionFormatter to
implement this interface.

Note that this is just the first of multiple steps to get these decoupled, with the goal
for this PR being to separate the two trees. I'm deferring the following to later PRs:
- Actually moving the expression classes into ksql-execution (this pollutes the diff).
- We have lots of AstVisitor implementations that only operate on expressions
(e.g. codegen, expression type manager, insert-into analyzer). Migrating these
to implement ExpressionVisitor is deferred to a later PR.
- The current patch has AstVisitor implement ExpressionVisitor. These should be
decoupled into separate visitors (and all made private inner classes (see below)),
and explicitly composed where needed.
- ExpressionVisitor is an interface, which means that all the visit methods are public,
but that's fine since the final implementation should always be a private inner class
so that the interface can be restricted appropriately for the given visitor use case.
Migrating AstVisitor (and cleaning up the weird inheritance hierarchy) to follow a
similar pattern is also deferred.

Testing done

This is purely a refactor, so no new tests have been added.

[agavra]

LGTM! Thanks for the detailed description :) helped answer questions