housekeeping: add .DS_Store files to .gitignore

[Qonstrukt]

What kind of change does this PR introduce? (Bug fix, feature, docs update, ...)
Make life easier for Mac users.

What is the current behavior? (You can also link to an open issue here)
.DS_Store files generated by macOS constantly screw up your git status.

What is the new behavior (if this is a feature change)?
Ignore .DS_Store files, since they're of no interest.

Does this PR introduce a breaking change?
Nope

Please check if the PR fulfills these requirements

• The commit follows our guidelines: https://github.com/reactiveui/reactiveui#contribute
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

Other information:

[ghuntley]

No idea why this didn't exist before. Don't merge Qconstruct; I'll run you through the process when it goes green.

[ghuntley]

Alright here's the process, run through it and self merge.

https://docs.reactiveui.net/en/contributing/framework/merging-pull-requests.html

Specifically make sure you use squash and rebase commits and the commit message matches conventions.

eg...

housekeeping: added dsstore to gitignore

[coveralls]

Coverage remained the same at 66.835% when pulling 1a361bb on Qonstrukt:dsstore-gitignore into cfbf11f on reactiveui:develop.