fix: Update yargs and prevent npm audit deps failures on CI

[rpl]

This PR contains:

• update yargs to version v15.3.1 (supersedes fix(deps): update dependency yargs to v15.3.1 - autoclosed #1859, and also seems to fix Yargs turns values like "--something" passed to web-ext run --args="..." as additional web-ext options  #1846 due to upstream changes)
• removes conventional-changelog-cli (a devDependency that isn't actually used, we just missed to remove it when we migrated to new commitlint packages)
• temporarily add yargs-related advisory to nsprc to prevent failures on all CI jobs (unfortunately commitlint didn't released an update yet, but after this change none of the dependencies uses the vulnerable yargs-parser version and so web-ext is not going to propagate the vulnerable yargs-parser version to projects that use web-ext as an npm dependency)

[coveralls]

Coverage remained the same at 100.0% when pulling 49c8932 on rpl:fix/npm-audit-deps-ci-failure-20200506 into 8c88b53 on mozilla:master.