feat(eslint-plugin): [strict-bool-expr] add allowSafe option

[Rajin9601]

Fixes #1025

allowSafe allows non-falsey types (i.e. non string / number / boolean) to be used as boolean expressions.

This option allows user to safely check null & undefined without having to explicitly write obj == null when used with allowNullable.

// as-is
const str = obj == null && obj.foo == null && obj.foo.bar

// to-be
const str = obj && obj.foo && obj.foo.bar

Note that setting allowNullable & allowSafe to true allows boolean | null as a boolean expression, which is not safe.

[typescript-eslint]

Thanks for the PR, @Rajin9601!

typescript-eslint is a 100% community driven project, and we are incredibly grateful that you are contributing to that community.

The core maintainers work on this in their personal time, so please understand that it may not be possible for them to review your work immediately.

Thanks again!

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[armano2]

LGTM

[bradzacher]

thanks for this!