Previously, if a matching rootDir ended with a slash then the path
returned from `logicalPathOfFile()` would not start with a slash,
which is inconsistent.
PR Close#29627
This commit introduces a new ngtsc sub-library, 'path', which contains
branded string types for the different kind of paths that ngtsc manipulates.
Having static types for these paths will reduce the number of path-related
bugs (especially on Windows) and will eliminate unnecessary defensive
normalizing.
See the README.md file for more detail.
PR Close#28523
