refactor: upgrade eslint and fix lint and type errors · vuepress/core@2d377d2

'app' is defined but never used

'router' is defined but never used

Unsafe assignment of an error typed value

Unsafe assignment of an error typed value

Unsafe assignment of an error typed value

Unsafe assignment of an error typed value

Functions that return promises must be async

Functions that return promises must be async

Template literal expression is unnecessary and can be simplified

Template literal expression is unnecessary and can be simplified

Unexpected console statement

Unexpected dangling '_' in '__VUEPRESS_VERSION__'

Unexpected dangling '_' in '__VUEPRESS_BASE__'

Unexpected dangling '_' in '__VUEPRESS_DEV__'

Unexpected dangling '_' in '__VUEPRESS_SSR__'

Unexpected dangling '_' in '__VUE_HMR_RUNTIME__'

Unexpected dangling '_' in '__VUE_OPTIONS_API__'

Unexpected dangling '_' in '__VUE_PROD_DEVTOOLS__'

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