Support https://typescript-eslint.io/rules/unbound-method in `lit/no-template-arrow` rule

This is similar to https://github.com/jest-community/eslint-plugin-jest/blob/main/src/rules/unbound-method.ts

The jest plugin checks whether the unbound method is valid in the context of jest, or it falls back to the typescript-eslint/unbound-method rule.

It would be helpful if the `lit/no-template-arrow` rule did the same. Especially now that the latest `eslint-config-standard-with-typescript` version now includes the `@typescript-eslint/unbound-method` rule.

Look at the existing `lit/no-template-arrow` rule implementation and the linked jest plugin rule (`src/rules/unbound-method.ts`). Understand how the jest plugin defers to `@typescript-eslint/unbound-method`. Modify the lit rule to check if the unbound method is valid in lit-html template contexts, otherwise fall back. Test with the latest `eslint-config-standard-with-typescript`.

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