refactor: improve modern middleware and spa modern rendering

[clarkdo]

Types of changes

• Bug fix (a non-breaking change which fixes an issue)
• New feature (a non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Description

Checklist:

• My change requires a change to the documentation.
• I have updated the documentation accordingly. (PR: #)
• I have added tests to cover my changes (if not applicable, please state why)
• All new and existing tests are passing.

[codecov-io]

Codecov Report

Merging #5037 into dev will decrease coverage by 0.01%.
The diff coverage is 95.45%.

@@            Coverage Diff             @@
##              dev    #5037      +/-   ##
==========================================
- Coverage   95.08%   95.07%   -0.02%     
==========================================
  Files          72       72              
  Lines        2421     2415       -6     
  Branches      614      615       +1     
==========================================
- Hits         2302     2296       -6     
+ Misses         99       98       -1     
- Partials       20       21       +1

Impacted Files	Coverage Δ
packages/server/src/server.js	94.25% <100%> (ø)	⬆️
packages/server/src/middleware/modern.js	96.87% <100%> (+4.56%)	⬆️
packages/vue-renderer/src/renderer.js	94.81% <100%> (-1.04%)	⬇️
packages/vue-renderer/src/spa-meta.js	86.2% <90.9%> (+0.24%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 8ea0f86...b81af31. Read the comment docs.

[awronski]

Off Topic: The modern browser list is now hardcoded.

Is there a risk, that some modern future is supported for example in Chrome 61 but not in Edge 16?
Or do you use the same list when transpiling the modern version (can't find it in the source code)?

[clarkdo]

@awronski The list is from babel-preset-env, the modern build is equal to babel-preset-env with esMdoules: true, so there is not conflict between them, the generated bundles are exact compatible with browsers in the list.

Building: build with babel-preset-env (modern browser list) -> generate modern bundles
Rendering: detect modern browsers (same browser list)-> serve modern bundles