chore(texttospeech): modernize quickstart for texttospeech

[iennae]

Description

Fixes #3504

(draft, still requires test update, region tags not added yet intentionally)

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Checklist

• I have followed guidelines from CONTRIBUTING.MD and Samples Style Guide
• Tests pass: npm test (see Testing)
• Lint pass: npm run lint (see Style)
• This pull request is from a branch created directly off of GoogleCloudPlatform/nodejs-docs-samples. Not a fork.
• Please merge this PR for me once it is approved

[grayside]

I found no blockers, but left several comments with opportunities for additional minor improvements. Offering them because I see the scope as "modernizing" the quickstart, which leaves the scope ambiguously larger than the referenced issue.

[subfuzion]

Hi @iennae, @grayside - I'm going to help with this as part of clearing the backlog.

[subfuzion]

@iennae / @grayside I went ahead and did a bunch of refactoring (feel free to undo with a git reset --hard @~1; git push -f). Still room for improvement, but makes things a bit more consistent and idiomatic. I did mention that we should standardize on Node's native support for testing back in early May, but for this PR I did remove chai for standard assert/strict. We should talk a bit more about swapping out test runners later. I'm going to slam this through as part of clearing the backlog.