Initial wip implementation of new code excerpter

[parlough]

This new package supports similar functionality to the old excerpter, with some functionality copied over, except it runs in one pass without package:build_runner with slightly less repetitive string processing. It is structured a bit differently to enable its use as a library package, instead of just as a command-line tool. Some functionality is simplified, changed, or removed, including:

• The unnamed region can't be changed. It's always the entire file.
• Region names must be alphanumerical with dashes.
• No support for Liquid prettify tags.
• No special support for Angular syntax.
• Transform order is always in the order provided.
• The region extractor and updater are more both strict, erroring or warning in cases the old implementations silently accepted, such as unclosed regions.
• Diffs are not yet supported.
• A few more comments for our future reference :)

There is more cleanup, implementation, and automatic testing to do, but I wanted to get this initial implementation up so other's can provide feedback and help complete that =D

As mentioned, diffs are not yet supported. This will be added later for diff outputs as well as for displaying what changed when running the updater.

Contributes to dart-lang/site-www#5593

[johnpryan]

This is a TON of cleanup, great work @parlough!

[johnpryan]

Looks great!