Rename semantic model methods to use current_* prefix (#6347)

## Summary

This PR attempts to draw a clearer divide between "methods that take
(e.g.) an expression or statement as input" and "methods that rely on
the _current_ expression or statement" in the semantic model, by
renaming methods like `stmt()` to `current_statement()`.

This had led to confusion in the past. For example, prior to this PR, we
had `scope()` (which returns the current scope), and `parent_scope`,
which returns the parent _of a scope that's passed in_. Now, the API is
clearer: `current_scope` returns the current scope, and `parent_scope`
takes a scope as argument and returns its parent.

Per above, I also changed `stmt` to `statement` and `expr` to
`expression`.

crates/ruff_python_semantic/src/binding.rs

@@ -185,7 +185,7 @@ impl<'a> Binding<'a> {
     /// Returns the range of the binding's parent.
     pub fn parent_range(&self, semantic: &SemanticModel) -> Option<TextRange> {
         self.source
-            .map(|node_id| semantic.stmts[node_id])
+            .map(|node_id| semantic.statements[node_id])
             .and_then(|parent| {
                 if parent.is_import_from_stmt() {
                     Some(parent.range())