Allow .log directories in Git and Perforce workspaces

[Dragon31337]

Summary

• Add a directory-only exception to Streamline's existing .gitignore and a root .p4ignore.
• Allow source/core/sl.log/ to be added when a parent project ignores *.log.
• Keep ordinary .log files ignored.

Validation

• Verified that a parent *.log rule ignores source/core/sl.log/ without the exception.
• Verified that !*.log/ re-includes the directory and its contents for Git.
• Verified with p4 ignores -i -v that the nested .p4ignore overrides the parent rule for sl.log/, while an ordinary example.log remains ignored.

Fixes #121