Add links to files in `internals/parser.rst`

[ezio-melotti]

This comment by @CAM-Gerlach suggested to add links to a few files mentioned in internals/parser.rst.

Even though direct links could be added, the page already refers to several other files using the :file: role, which doesn't create any link.

Maybe a new role could solve that.

[ezio-melotti]

#961 adds a new role to link to cpython files. The links are also checked by linkcheck automatically. Based on the work @hugovk did in #932.