Move python into its own ruff_python crate (#2593)

crates/ruff_python/src/identifiers.rs

@@ -0,0 +1,24 @@
+/// Returns `true` if a string is a valid Python identifier (e.g., variable
+/// name).
+pub fn is_identifier(s: &str) -> bool {
+    // Is the first character a letter or underscore?
+    if !s
+        .chars()
+        .next()
+        .map_or(false, |c| c.is_alphabetic() || c == '_')
+    {
+        return false;
+    }
+
+    // Are the rest of the characters letters, digits, or underscores?
+    s.chars().skip(1).all(|c| c.is_alphanumeric() || c == '_')
+}
+
+/// Returns `true` if a string is a private identifier, such that, when the
+/// identifier is defined in a class definition, it will be mangled prior to
+/// code generation.
+///
+/// See: <https://docs.python.org/3.5/reference/expressions.html?highlight=mangling#index-5>.
+pub fn is_mangled_private(id: &str) -> bool {
+    id.starts_with("__") && !id.ends_with("__")
+}