When running `npm test`, skip coveralls when not configured

[jwhitlock]

Add a new script scripts/is_coveralls_configured.js that checks if COVERALLS_REPO_TOKEN is defined or .coveralls.yml exists, and sets exit code 1 (failure) if not configured. It can be run with npm test:is_coveralls_configured

This is then used in npm test to skip running coveralls if it isn't configured, which is normal in the development environment.

[jwhitlock]

Force-pushed to fix the command. coveralls ran in CircleCI and coverage was uploaded to coveralls.io.

It is skipped for me locally (since I never setup .coverage.yml).

We're missing a coverage badge. I could add it to README.md with this PR:

[![Coverage Status](https://coveralls.io/repos/github/mozilla/blurts-server/badge.svg?branch=master)](https://coveralls.io/github/mozilla/blurts-server?branch=master)

which renders as:

[jwhitlock]

Rebased on current main and made changes based on feedback:

• Script is now scripts/is-coveralls-configured.js, using kabob-naming for consistency with other scripts
• npm test now calls scripts/is_coveralls_configured.js directly, and npm test:is_coveralls_configured is dropped
• Remove a comment about .coverage.yml and the Ruby coveralls as unneeded
• Remove unneeded call to process.exit(0), only needed for non-zero exit code
• Add the coverage badge to README.md, defaulting to the main branch (not master, like in my comment)