Debug events

[RedHatter]

This pull request causes code bundles generated with the dev flag on to emit events on document when various things happen.

• Blocks are created
• Components classes are created
• Any DOM multiplication

It also adds a Component.$unsafe_set function that allows any state to be set from outside of the component.

This would open up the possibility for tools to be created that "plug into" svelte. One such tools is Svelte DevTools that I've been working on. Another might be a svelte aware testing framework.

See also #2931

[RedHatter]

@Rich-Harris Not to be a bother, but any chance you can review this sometime soon? I can't move forward with svelte-devtools without it.

[codecov-io]

Codecov Report

Merging #3005 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #3005   +/-   ##
=======================================
  Coverage   50.25%   50.25%           
=======================================
  Files           1        1           
  Lines         197      197           
=======================================
  Hits           99       99           
  Misses         98       98

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[Rich-Harris]

Thanks so much for this PR, and sincere apologies for the length of time it's taken to review it (once the PR backlog grows past a certain size I fall into the trap of prioritising 'quick wins' and urgent bug fixes over more structural things that require more attention. I know that's annoying for everyone else, and I'm trying to get better at it 😬).

This all looks great to me. I've brought it up to date with the current master. I made one change — since merging #3148 we now have an $inject_state method which is very similar to $unsafe_set. There was just one small behavioural difference ($inject_state only updates state that can change, whereas for devtools $unsafe_set could change all writable variables), so I've updated $inject_state to behave more like $unsafe_set, and removed the latter (cc @Axelen123)

If you're good with that change, then I believe this is ready to merge.

[RedHatter]

Looks great. Thanks for the review

[Rich-Harris]

Perfect — released 3.12.0. Thanks again