Add unbound-method rule

[FloEdelmann]

See jest/unbound-method.

The rule source does already exist in this repo, but its tests are skipped and naively adding it to index.ts yields an error:

/home/flo/www/eslint-plugin-vitest/src/rules/unbound-method.ts:26
    const TSESLintPlugin = require('@typescript-eslint/eslint-plugin')
                           ^

ReferenceError: require is not defined in ES module scope, you can use import instead

[veritem]

it's partially done, I haven't got time to finish it.