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

首先梳理现有的 JavaScript/WASM 桥接层以及列出的入口点：swjs_core、swjs_object、swjs_string、swjs_function 和 swjs_typedarray。确定公共 plugin API 的边界，以及列出的 JavaScriptKit 功能可以如何使用它；完成的标准是设计已经确定，并且可以迁移提议的函数，而无需向每个 bundle 添加未使用的功能。