fix(slides): check that mutation observer is defined for ssr

[brandyscarney]

Pull request checklist

Please check if your PR fulfills the following requirements:

• Tests for the changes have been added (for bug fixes / features)
• Docs have been reviewed and added / updated if needed (for bug fixes / features)
• Build (npm run build) was run locally and any changes were pushed
• Lint (npm run lint) has passed locally and any fixes were made for failures

Pull request type

Please check the type of change your PR introduces:

• Bugfix
• Feature
• Code style update (formatting, renaming)
• Refactoring (no functional changes, no api changes)
• Build related changes
• Documentation content changes
• Other (please describe):

What is the current behavior?

Issue Number: N/A

When running stencil build --prod in the stencil conference app the following error happens prior to this PR:

[12:28.5]  prerendering failed in 4.75
           s

[ ERROR ]  Hydrate Error: /tutorial
           ReferenceError: MutationObserver is not defined at
           Slides.connectedCallback

What is the new behavior?

• This fixes that error on the prerender

Does this introduce a breaking change?

• Yes
• No

Other information