Modules: introduced QuickJS engine.

Modules: introduced QuickJS engine.

"js_engine" directive is introduced which sets JavaScript engine.
When the module is built with QuickJS library "js_engine qjs;" sets
QuickJS engine for the current block. By default njs engine is used.

For example,

nginx.conf:

    location /a {
        js_engine qjs;
        # will be handled by QuickJS
        js_content main.handler;
    }

    location /b {
        # will be handled by njs
        js_content main.handler;
    }

QuickJS engine implements drop-in replacement for nginx/njs objects
with the following exceptions:

    * nginx module API to be added later: ngx.fetch(), ngx.shared.dict.
    * Built-in modules to be added later: fs, crypto, WebCrypto, xml.
    * NJS specific API: njs.dump(), njs.on(), console.dump().
    * js_preload_object directive.