Allow parsing to `Expr`-structured `SyntaxTree` (#60474)
This change allows parsing to "`EST`" (`SyntaxTree` with Expr structure
for compatibility with macros) by adding a way to convert `RawGreenNode` to
`EST`. This is pretty similar to our existing `RawGreenNode`->`Expr` conversion
`node_to_expr`, so there's some unfortunate duplication here, but I did
this over making `node_to_expr`'s output generic for a few reasons:
- `RawGreenNode`->`Expr` is production code we're using today and is the
reference I'm testing this change against, so I don't want to screw with it.
- Producing `SyntaxTree` means working with different constraints:
- We can't efficiently mutate the list of children after the tree is created.
- Nodes that are created and never used will stick around in the graph.
- Each node needs to remember what node it was converted from.
- I've cleaned up the algorithm by postponing recursion on child nodes
until after we decide what child nodes there will be in most cases.
- In a future where we pass provenance from parsing to macro-expansion
by default, we can just delete the old conversion and use
`RawGreenNode`->`EST`->`Expr` in its place.
