Add GitHub actions

[brettcannon]

Description (e.g. "Related to ...", etc.)

Introduce GitHub Actions for CI. This will run all tests and linting on each PR and push to the repository.

It will also build the example JSON extension as appropriate. This required introducing an ESLint configuration file which checks for errors and common mistakes.

Closes #172

Code review checklist (for code reviewer to complete)

• Pull request represents a single change (i.e. not fixing disparate/unrelated things in a single PR)
• Title summarizes what is changing
• Commit messages are meaningful (see this for details)
• Tests have been included and/or updated, as appropriate
• Docstrings have been included and/or updated, as appropriate
• Standalone docs have been updated accordingly
• CONTRIBUTORS.md was updated, as appropriate
• Changelog has been updated, as needed (see CHANGELOG.md)

[brettcannon]

Demo of the workflows running: https://github.com/brettcannon/pygls/actions.

[brettcannon]

I also didn't remove the Azure Pipelines configuration as I wasn't sure if you wanted to keep both, remove it in this PR, remove it in a separate PR, etc.

[danixeee]

Looks good overall.

I noticed that the json-extension vsix artifact is not uploaded after successful build. Could you check it?

No files were found with the provided path: *.vsix. No artifacts will be uploaded.

[brettcannon]

I addressed the review comments and made it an error if no extension gets uploaded.

[danixeee]

Looks good now, I'll remove Azure Pipelines. Thanks!