Update eslint

[AndrewFerr]

This PR removes the following warning during yarn lint:

WARNING: You are currently running a version of TypeScript which is not officially supported by @typescript-eslint/typescript-estree.

You may find that it works just fine, or you may not.

SUPPORTED TYPESCRIPT VERSIONS: >=3.3.1 <5.1.0

YOUR TYPESCRIPT VERSION: 5.1.3

Please only submit bug reports when using the officially supported version.

For posterity, the (bash) command I used to update the eslint deps is

yarn add -D @typescript-eslint/{eslint-plugin,parser} eslint{,-{config-preact,plugin-mocha}}