New: filepermissions.chmod

[juancarlospaco]

• Improve filepermissions with a convenience proc to save some typing.
• More familiar and friendly API for end users chmod "file.ext", 0o666

:)

[timotheecour]

• filename seems wrong, os.nim uses convention: file: a file name (can include dirSep) dir: a dir name filename: a file name without dirSep path: a file or dir

• what about directories? should this be named chmod(path: string, ...) to cover both?

• what about sticky bit (+ setuid, setgid)? this is probably out of scope for this PR since it'd require changing os.nim

• for making it easier to port shell scripts to nim, I'd also like to have: chmod(path: string, perms: string), and in particular support: chmod("a1.txt", "u+x"), which simplifies user code. This could be done in future work though

[juancarlospaco]

@timotheecour Feel free to send pull request to add Sticky bits to os.nim, then that feature can be added here too, I dont know how to do it.

[timotheecour]

@juancarlospaco I've merged this but can you please follow-up with a changelog.md entry?