refactor(vue-renderer): split renderer into ssr, spa and modern

[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

Changes:

• split renderer functionality into ssr, spa and modern renderer, renderer is for managing renderers, resources/template and ready state, the specific ssr,spa,modern renderer is for app rendering.
• change req.modernMode to req._modern for more explicit to indicate request status
• move modern build detection to renderer for getting correct modern mode in renderer initialization.

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.

[pi0]

Really nice refactors @clarkdo ⚡ My comments are further than the refactor but can be cool if we can have them :)

[clarkdo]

@pi0 Refactored as code review comments

changes applied

[pi0]

This has no coverage as we are internally always implementing these functions. Is it necessary to keep? @clarkdo

[clarkdo]

At least keep the function signature for consistent common definition？

[pi0]

JS is not a typed language to expect defining interface. Maybe can be useful for typescript or ts typings. For coverage, we can either ignore the line or write extra internal tests :)

[clarkdo]

Not for type, just for reminding contributors which are gonna implement new renderer.
Yes, could be moved to ts types.

[codecov-io]

Codecov Report

Merging #5559 into dev will decrease coverage by 0.05%.
The diff coverage is 96.49%.

@@            Coverage Diff             @@
##              dev    #5559      +/-   ##
==========================================
- Coverage   95.68%   95.62%   -0.06%     
==========================================
  Files          78       81       +3     
  Lines        2595     2608      +13     
  Branches      661      661              
==========================================
+ Hits         2483     2494      +11     
- Misses         93       95       +2     
  Partials       19       19

Impacted Files	Coverage Δ
packages/server/src/middleware/nuxt.js	97.33% <ø> (-0.04%)	⬇️
packages/server/src/middleware/modern.js	100% <100%> (ø)	⬆️
packages/webpack/src/builder.js	93.54% <100%> (ø)	⬆️
packages/server/src/server.js	97.72% <100%> (ø)	⬆️
packages/vue-renderer/src/renderers/spa.js	87.3% <100%> (ø)
packages/vue-renderer/src/renderers/base.js	77.77% <77.77%> (ø)
packages/vue-renderer/src/renderer.js	93.44% <95.83%> (-1.68%)	⬇️
packages/vue-renderer/src/renderers/ssr.js	97.56% <97.56%> (ø)
packages/vue-renderer/src/renderers/modern.js	98.21% <98.21%> (ø)
... and 2 more

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