Periodically check client pid and exit server

[danixeee]

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

Closes #118, #130.

Ensures that pygls process will terminate if the client process is not alive.

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)

[danixeee]

@dimbleby This was a quick fix, I will release updates tomorrow. You can also check this PR if you wish.

[renatav]

Just one typo. Looks good to me