Provide a modular build of the browser SDK that does not bundle the plugins

[stof]

Summary

BrowserClient currently defines an API allowing to enable or disable autocapture features through boolean options. This implies that it has to bundle the associated plugins even if the options will disable them.

A modular API should be provided, supporting the code features for the browser, but without any bundled plugins. This will give choices to projects:

1. use the modular core and register wanted plugins explicitly
2. use the exist BrowserClient that would use that modular core and keep bundling the plugins for ease of use (maybe excluding experimental plugins from the bundle plugins, requiring them to be registered directly).

See #325 (comment) for a comment (by me) proposing this solution to the bundle size bloat issue.
This modular core would at least solve this partially (it won't help much projects using autocapture).

Motivations

The bundle size of @amplitude/analytics-browser is quite big, and tends to grow bigger when adding new autocapture plugins.
When not using autocapture (which is my case), half of that bundle size is wasted by autocapture-related plugins

[daniel-graham-amplitude]

Thanks @stof . I'll add this to our backlog as a feature request.