Abstract stylist to libcst style conversion (#4749)
* Abstract codegen with stylist into a CodegenStylist trait
Replace all duplicate invocations of
```rust
let mut state = CodegenState {
default_newline: &stylist.line_ending(),
default_indent: stylist.indentation(),
..CodegenState::default()
}
tree.codegen(&mut state);
state.to_string()
```
with
```rust
tree.codegen_stylist(&stylist);
```
No functional changes.
