Upgrade Mochify to new @mochify/cli powered by esbuild

[fatso83]

Similarly to other Sinon projects, we should upgrade Mochify to get security fixes and a nice speed bump in bundling and running tests (not that we need it: we run 600 tests in 300 ms).

[fatso83]

❯ npm audit
# npm audit report

ws  8.0.0 - 8.17.0
Severity: high
ws affected by a DoS when handling a request with many HTTP headers - https://github.com/advisories/GHSA-3h5v-q93c-6h6q
fix available via `npm audit fix --force`
Will install mochify@9.1.0, which is a breaking change
node_modules/puppeteer/node_modules/ws
  puppeteer  11.0.0 - 18.1.0
  Depends on vulnerable versions of ws
  node_modules/puppeteer
    mochify  >=9.2.0
    Depends on vulnerable versions of puppeteer
    node_modules/mochify

3 high severity vulnerabilities