chore: initial flow setup

[faustbrian]

Proposed changes

Initial setup of https://flow.org to get the benefits of static type checking as we lately had some issues that would have shown up right away with it.

All the current errors will be fixed in subsequent PRs, this is just the setup.

Types of changes

• Other

Checklist

• I have read the CONTRIBUTING documentation
• Lint and unit tests pass locally with my changes

[codecov-io]

Codecov Report

Merging #1364 into develop will increase coverage by 0.13%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           develop    #1364      +/-   ##
===========================================
+ Coverage     73.7%   73.83%   +0.13%     
===========================================
  Files          419      419              
  Lines         7431     7431              
  Branches      1008     1008              
===========================================
+ Hits          5477     5487      +10     
+ Misses        1720     1713       -7     
+ Partials       234      231       -3

Impacted Files	Coverage Δ
...p2p/lib/server/versions/internal/handlers/utils.js	86.66% <0%> (+6.66%)	⬆️
packages/core-forger/lib/manager.js	79.59% <0%> (+9.18%)	⬆️

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