spec/code_spec.rb: refactor tests

- Use fixtures as a test files
- Write more concise spec titles
spec/code_spec.rb: add more tests for $LOAD_PATH behaviour
lib/pry/code.rb: remove #known_extensions cruft
lib/pry/code.rb: factor out code related to filename handling

Factor out it to `lib/pry/code/code_file.rb`.
lib/pry/code.rb,lib/pry/code/code_file.rb: add docs