Plugin infrastructure

Currently, JavaScript-based features must be directly implemented in JavaScriptKit to be able to access important features like the heap and the JSValue parsing/serializing routines. This means that less-common things like TypedArray or potential Combine-Promise integration (see #62) would have to increase the bundle size for everyone. One solution to this would be defining some sort of public JS-side API that allows providing custom functions to the WASM runtime. As long as we encourage users of this API to prefix their methods there should be no collision issues.

It would be nice to restructure JavaScriptKit itself to make use of the plugin API too:

• swjs_core: release
• swjs_object: get_prop, set_prop, get_subscript, set_subscript, instanceof
• swjs_string: decode, encode, load
• swjs_function: call, apply, call_new, create
• swjs_typedarray: create

首先整理現有的 JavaScript/WASM 橋接層以及列出的入口點：swjs_core、swjs_object、swjs_string、swjs_function 和 swjs_typedarray。確定公開 plugin API 的邊界，以及列出的 JavaScriptKit 功能可以如何使用它；完成的標準是設計已經確定，並且可以遷移提議的函式，而不必向每個 bundle 新增未使用的功能。