feat: support multiple apps on one page

[pimlie]

Resolves: #372

Problems with this implementation:

• have no good solution found yet for html, head, body attributes
• although vmid's work across app's, support is only partial
  • the value of the last refreshed app is used
  • given app1 and app2 where app2 removes its value for vmid1, then there is no way to retrieve app1's value for vmid1 which means the element is removed

The new multi app example renders as follows:

[codecov-io]

Codecov Report

Merging #373 into master will decrease coverage by 0.97%.
The diff coverage is 95.23%.

@@            Coverage Diff             @@
##           master     #373      +/-   ##
==========================================
- Coverage     100%   99.02%   -0.98%     
==========================================
  Files          30       30              
  Lines         398      409      +11     
  Branches      112      115       +3     
==========================================
+ Hits          398      405       +7     
- Misses          0        4       +4

Impacted Files	Coverage Δ
src/server/generators/tag.js	100% <ø> (ø)	⬆️
src/client/updaters/tag.js	100% <100%> (ø)	⬆️
src/client/updateClientMetaInfo.js	92.85% <100%> (-7.15%)	⬇️
src/server/generators/title.js	100% <100%> (ø)	⬆️
src/client/$meta.js	100% <100%> (ø)	⬆️
src/server/generateServerInjector.js	100% <100%> (ø)	⬆️
src/client/refresh.js	100% <100%> (ø)	⬆️
src/shared/constants.js	100% <100%> (ø)	⬆️
src/server/inject.js	100% <100%> (ø)	⬆️
src/server/$meta.js	100% <100%> (ø)	⬆️
... and 2 more

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