Add release workflow

[Aloso]

Adds a GitHub workflow that creates a new release and builds binaries for Linux, Windows and Mac whenever a new git tag is pushed.

Use it like this:

git tag v2.0   # create a tag for version 2.0
git push v2.0  # push the tag

This will trigger the workflow. After a moment, a new release should appear on https://github.com/adam-mcdaniel/dune/releases. The text of the release can be edited manually.

When the workflow is finished, the release should include artifacts, which are binaries for Windows, Linux and MacOS. If this should fail for some reason, the artifacts can also be added manually.

[Aloso]

@adam-mcdaniel the workflow worked and created a release for version 0.1.7. However, there are two problems:

• For some reason the tag isn't included in the name of the Windows executable. It can be manually renamed though.
• The link to the changelog is dead since the PR adding the changelog isn't merged yet. When the changelog is merged, the link will have to be modified, since the URL refers to the v0.1.7 tag specifically.