feat: Export `getCanvasManager` & allow passing it to `record()`

[mydea]

This PR exports a new getCanvasManager() method which can be used to dynamically pass a canvas manager, allowing tree shaking.

This also removes the __RRWEB_EXCLUDE_CANVAS__ build flag - canvas will always be excluded now by default.

Expected usage:

import { record, getCanvasManager } from '@sentry-internal/canvas';

record({
  // other config...
  getCanvasManager,
});

The idea is that we can expose this somehow (?) from replay, so users can do e.g.:

import { Replay, getReplayCanvasManager } from '@sentry/browser';

Sentry.init({
  integrations: [
    new Replay({ canvasManager: getReplayCanvasManager() })
  ]
});

Or something like this, allowing people to opt-in to canvas recording at runtime, vs requiring a specific build step for it.

[mydea]

Can we make this default? (e.g. get rid of build flag?)

Yeah, I guess that makes sense here! 👍

[github-actions[bot]]

size-limit report 📦

Path	Size
rrweb - record only (gzipped)	16.6 KB (-26.27% 🔽)
rrweb - record only (min)	57.17 KB (-27.45% 🔽)
rrweb - record with treeshaking flags (gzipped)	15.35 KB (+0.22% 🔺)
rrweb - record & getCanvasManager only (gzipped)	22.54 KB (added)

[mydea]

OK, I fixed the tests, seems good now. I ended up changing it so that you have to pass in a getCanvasManager method instead of the instance, which is a slightly "worse" API, but easier to implement because we need access to some of the parsed options from record, which we don't easily have otherwise 😬