feat: Added expose to component instances

[baiwusanyu-c]

close: #74
relate: #73
https://github.com/vuejs/core-vapor/pull/73/files#diff-4cb2e944d7a507fecd850ac73c52ca8e2cca9685136ad3c3d4c9d1e0646a510aR408

[github-actions]

Size Report

Bundles

File	Size	Gzip	Brotli
runtime-dom.global.prod.js	89.5 kB (+20 B)	34 kB (+5 B)	30.7 kB (-2 B)
runtime-vapor.global.prod.js	28.8 kB (+5.53 kB)	10.7 kB (+1.88 kB)	9.83 kB (+1.65 kB)
vue-vapor.global.prod.js	63.7 kB (+5.53 kB)	22.2 kB (+2.3 kB)	19.8 kB (+1.69 kB)
vue.global.prod.js	147 kB (+20 B)	53.3 kB (+7 B)	47.6 kB (+3 B)

Usages

Name	Size	Gzip	Brotli
createApp	49.8 kB	19.5 kB	17.8 kB
createSSRApp	53.2 kB	20.8 kB	19 kB
defineCustomElement	52.1 kB	20.3 kB	18.5 kB
vapor	29.5 kB (+6.35 kB)	11.1 kB (+2.2 kB)	10.2 kB (+1.98 kB)
overall	63.3 kB	24.5 kB	22.2 kB

[baiwusanyu-c]

I noticed that the review is marked as resolved, is there anything else I need to do, or is the PR no longer useful and I need to close it? @sxzz
In addition, the proxy on the component instance has been deleted(efc32b7). I am not sure whether this is a design consideration. This will cause some use cases of this PR and related tests to fail.