Add a dedicated token indexer for continuations and comments (#1886)

The primary motivation is that we can now robustly detect `\` continuations due to the addition of `Tok::NonLogicalNewline`. This PR generalizes the approach we took to comments (track all lines that contain any comments), and applies it to continuations too.

src/source_code/mod.rs

@@ -1,8 +1,10 @@
 mod generator;
+mod indexer;
 mod locator;
 mod stylist;
 
 pub(crate) use generator::Generator;
+pub(crate) use indexer::Indexer;
 pub(crate) use locator::Locator;
 use rustpython_parser::error::ParseError;
 use rustpython_parser::parser;