Permit plugins to add assets directly to the bundle in Rollup 1.x

[lukastaegert]

This PR contains:

• bugfix
• feature
• refactor
• documentation
• other

Are tests included?

• yes (bugfixes and features will not be merged without tests)
• no

Breaking Changes?

• yes (breaking changes will not be merged unless absolutely necessary)
• no

List any relevant issue numbers:
Resolves #3104

Description

Apparently some plugins heavily depend on the ability to directly add assets to the bundle object. This will permit this workflow again but add a deprecation message that will show when using strictDeprecations.

[codecov]

Codecov Report

Merging #3105 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #3105      +/-   ##
==========================================
+ Coverage   89.27%   89.28%   +<.01%     
==========================================
  Files         165      165              
  Lines        5726     5731       +5     
  Branches     1739     1740       +1     
==========================================
+ Hits         5112     5117       +5     
  Misses        380      380              
  Partials      234      234

Impacted Files	Coverage Δ
src/rollup/index.ts	93.81% <100%> (+0.16%)	⬆️

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[jackturnbull]

Thanks for the fix here - was scratching my head why my little manifest plugin was returning undefined. I wasn't aware of this.emitAsset so I'm glad of that.

[lukastaegert]

Actually the proper way is this.emitFile by now as this also allows to use arbitrary names if you need to :) https://rollupjs.org/guide/en/#thisemitfileemittedfile-emittedchunk--emittedasset--string

[lukastaegert]

And that's what I should have written, damn. Maybe another day...

[hiendv]

Should we also print the plugin adding properties? As an end-user, it surely helps tracking down the cause of the warning.