fix(require-explicit-slots): recognize slot names enclosed in quotes

[waynzh]

solves https://github.com/vuejs/eslint-plugin-vue/issues/2391

1. The update now allows node.key to be a string, supporting both Identifier and Literal types. It also skips over dynamic slot names since slotNameAttr only detects nodes with directive: false.

2. The memberNode type has been narrowed down to only include TSMethodSignature and TSPropertySignature.

• foo(): any
• foo: () => any