[V2] Drop NodeJS 8 support and test NodeJS 12.

[LoicPoullain]

Issue

Since the end of 2019, version 8 of Node is no longer maintained. There is no reason to keep maintaining its support in FoalTS version 2. We will able to use all features of Node 10, especially those on promises in the assert module (for unit testing).

Solution and steps

• Remove Node 8 from CI and test Node 12.
• Test Node 12 and make changes if necessary.
• Remove Node 8 from documentation.
• Add version warning in @foal/core.
• Update CLI to generate new project with Node 10 types.
• Test manually the deployment with version 10 on firebase (to check if the doc is correct).

Checklist

• Add/update/check docs (code comments and docs/ folder).
• Add/update/check tests.
• Update/check the cli generators.

[codecov]

Codecov Report

Merging #699 into v2-0-0 will decrease coverage by 0.05%.
The diff coverage is 80.00%.

@@            Coverage Diff             @@
##           v2-0-0     #699      +/-   ##
==========================================
- Coverage   99.65%   99.59%   -0.06%     
==========================================
  Files          94       94              
  Lines        1727     1732       +5     
  Branches      407      407              
==========================================
+ Hits         1721     1725       +4     
- Misses          6        7       +1     

Impacted Files	Coverage Δ
packages/core/src/index.ts	90.00% <80.00%> (-10.00%)	⬇️

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