Upgrade engine.node to 16.x, or 14.18.

[hyrious]

When I started this repo, node.js's Current LTS version is 14.x, and I radically use 14.8 to make it able to use top-level-await (from 14.8) and --enable-sourcemaps (from 12.12).

Taking these into consideration:

• 14.8 is such weird version that it does not support ??=, but supports #private_field
  evanw/esbuild#1493, note the live link in this issue will cause error the in console, change the ?version= in the address bar to some newer version to correct it
• --enable-sourcemaps is no longer a needed flag since 14.18
  https://nodejs.org/docs/latest-v17.x/api/cli.html#--enable-source-maps

Besides, this repo is a developing tool, so most of the time you will not use a very old version of node. (But you may use yarn1, which may block some users).

[hyrious]

Now it is >= 16.13.