fix(typescript): use Proxy instead of modifying LanguageService instance

[johnsoncodehk]

Referring to https://github.com/microsoft/typescript-template-language-service-decorator/pull/31, we found that some TS plugins use Proxy but do not implement the set method, which causes the method of decorateLanguageService to override LanguageService functions to be invalid.

This PR attempts to change decorateLanguageService to use Proxy to solve this problem without requiring modification of other TS plugins to improve Volar TS plugin compatibility.

cc @volarjs/core-team, @piotrtomiak (Please let me know if you think this is potentially breaking in WebStorm.)

[johnsoncodehk]

This makes the following TS plugins now compatible. 🎉 (From https://github.com/vuejs/language-tools/pull/4206)