We refactor Executable and StaticFile through a base class FileFeature.

The method absolute_filename replaces StaticFile.absolute_path and is now also available for Executable. (This is for #33440/#31296/#30818.)

The renaming from StaticFile.absolute_path to absolute_filename (with deprecation) is preparation for reclaiming the name absolute_path for a method that returns a Path instead of a str.

We also add some type annotations.

Follow-up: #31296

CC: @tobiasdiez @fchapoton @seblabbe @orlitzky @kiwifb @antonio-rojas @tornaria

Component: refactoring

Author: Matthias Koeppe

Branch/Commit: e9568e9

Reviewer: Tobias Diez

Issue created by migration from https://trac.sagemath.org/ticket/31292