- Updated 'ava' package version.

- Added '@ava/typescript' module to package (required by latest version of AVA).
- Updated tests to follow new syntax for 'after', and 'before, methods.

…wnlevel-dts', 'copy' steps for downlevel-dts output directory ('./ts3.5').

- Added generated 'downleveled dts' files (keeping this in the git cache
since we're already keeping the library's artifacts there - more
consistent).

…us commit.

… message.

- Ran auto format on 'clean-and-run' script.

- Updated 'SassImporterResult' type to match Sass's 'LegacyImporterResult' - We had defined our type because the version of sass the library using, and sass versions users might be using, did not/may not contain this type (so basically our type was that type but slightly imprecise (we had both 'file?:...', and 'content?:...'  as part of the same interface, but actually they need to be separate ones (:-))).