Split the usethis._tool.impl layer into usethis._tool.impl.spec and usethis._tool.impl.base layers #1356

Current dir structure looks like this:

src\usethis_tool\impl
src\usethis_tool\impl_init_.py
src\usethis_tool\impl\codespell.py
src\usethis_tool\impl\coverage_py.py
src\usethis_tool\impl\deptry.py
src\usethis_tool\impl\import_linter.py
src\usethis_tool\impl\mkdocs.py
src\usethis_tool\impl\pre_commit.py
src\usethis_tool\impl\pyproject_fmt.py
src\usethis_tool\impl\pyproject_toml.py
src\usethis_tool\impl\pytest.py
src\usethis_tool\impl\requirements_txt.py
src\usethis_tool\impl\ruff.py

Each module contains the implementation for a ToolSpec and a Tool subclass.

We should create a submodule in the impl module for spec and base separately with parallel submodule structures.

This helps us work toward #1311