Revisit our Typescript linting rules

[pokey]

• Are we not using typescript-eslint/recommended? If not, try it
• Ensure we're using no-unnecessary-type-assertion. Should come from recommended above
• Ensure we're using explicit-function-return-type. Might need to tweak config if it's too aggressive, eg in lambdas. See also Revisit our Typescript linting rules #714 (comment)
• Other best practices we're missing?
• Do we want to use / borrow from xo?
• eslint: Use consistent-type-imports #872 (closed due to not being super high priority, but still a good direction). Can reopen to finish
• Add default ruleset from https://github.com/sindresorhus/eslint-plugin-unicorn?
• Add other default rulesets?

Originally posted by @pokey in #705 (comment)

[auscompgeek]

I don't see where our eslintrc extends typescript-eslint/recommended. Maybe we have a whole bunch of lints and we just haven't noticed 👀

[pokey]

Updated description to be more general

[auscompgeek]

explicit-function-return-type

Should we use explicit-module-boundary-types rather than explicit-function-return-type?

[auscompgeek]

Unassigning myself because I realistically won't have time to enable more lint rules.

It'll be good to consider the lint rules that use type information, but we should consider how enabling type checking in linting affects the linter performance.