fix(types): support this type with inject option api

[rudyxu1102]

When using inject option api, I can't get foo in this type.

 defineComponent({
    props: ['a'],
    inject: {
      foo: 'foo'
    },
    created() {
      //  ts throw error：Property 'foo' does not exist 
      console.log(this.foo)
    }
  })

I create a PR that inferring this.foo as a unknown. Maybe can fix issues below.

close #5931
close #3031
link #5216

[yyx990803]

Good job!