feat(meta): merge head at runtime

[pi0]

For a long time, we had to always add @nuxtjs/pwa for production to fill options.meta (providing icons, etc).

This refactor, exports .nuxt/pwa/meta.json file which is merged at runtime using a nuxt plugin with user provided head. Also SPA meta support (+ static 200.html ) is preserved by loading same json file and using same merging strategy.

Note: Users using ssr: false in production mode, have to still use modules instead of buildModules

Fixes #341

[codecov]

Codecov Report

Merging #363 into master will decrease coverage by 4.97%.
The diff coverage is 65.45%.

@@            Coverage Diff             @@
##           master     #363      +/-   ##
==========================================
- Coverage   84.82%   79.84%   -4.98%     
==========================================
  Files          10       11       +1     
  Lines         369      387      +18     
  Branches      109      115       +6     
==========================================
- Hits          313      309       -4     
- Misses         52       66      +14     
- Partials        4       12       +8     

Impacted Files	Coverage Δ
lib/utils/index.js	88.46% <ø> (-0.83%)	⬇️
lib/meta/meta.merge.js	4.54% <4.54%> (ø)
lib/meta/module.js	79.09% <79.74%> (-1.68%)	⬇️
lib/manifest/module.js	93.10% <83.33%> (+1.92%)	⬆️
lib/icon/module.js	93.18% <100.00%> (+0.87%)	⬆️

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