This PR renames `String.endPos` to `String.rawEndPos`, as in a future
release the name `String.endPos` will be taken by the function that is
currently called `String.endValidPos`.
This PR adds the necessary infrastructure for recording elaboration
dependencies that may not be apparent from the resulting environment
such as notations and other metaprograms. An adapted version of `shake`
from Mathlib is added to `script/` but may be moved to another location
or repo in the future.