[NEW] New set of rules for client code

[tassoevan]

Proposed changes (including videos or screenshots)

This small PR does the following:

• Now React is the web client's first-class citizen, being loaded before Blaze. Thus, BlazeLayout calls render templates inside of a React component (BlazeLayoutWrapper);
• Main client startup code, including polyfills, is written in TypeScript;
• At the moment, routes are treated as regular startup code; it's expected that FlowRouter will be deprecated in favor of a new routing library;
• React was updated to major version 17, deprecating the usage of React as namespace (e.g. use memo() instead of React.memo());
• The client/ and ee/client/ directory are linted with a custom ESLint configuration that includes:
  • Prettier;
  • react-hooks/* rules for TypeScript files;
  • react/no-multi-comp, enforcing the rule of one single React component per module;
  • react/display-name, which enforces that React components must have a name for debugging;
  • import/named, avoiding broken named imports.
• A bunch of components were refactored to match the new ESLint rules.

Issue(s)

Steps to test or reproduce

Further comments

Target browser changes:

-and_chr 84
-and_ff 68
+and_chr 89
+and_ff 86
 and_qq 10.4
 and_uc 12.12
-android 81
+android 89
 baidu 7.12
 bb 10
 bb 7
-chrome 84
-chrome 83
-edge 84
-edge 83
-firefox 79
-firefox 78
+chrome 89
+chrome 88
+edge 89
+edge 88
+firefox 87
+firefox 86
 ie 11
 ie 10
 ie_mob 11
 ie_mob 10
-ios_saf 13.4-13.5
-ios_saf 13.3
+ios_saf 14.0-14.5
+ios_saf 13.4-13.7
 kaios 2.5
 op_mini all
-op_mob 46
+op_mob 62
 op_mob 12.1
-opera 69
-opera 68
+opera 73
+opera 72
+safari 14
 safari 13.1
-safari 13
+samsung 13.0
 samsung 12.0
-samsung 11.1-11.2

[ankar84]

Great job @tassoevan and @ggazzo you guys rocks!

[pythoneer]

@tassoevan @ggazzo it is a little bit unclear which versions this pull request have affected. This PR possibly broke functionality explained at #22098 (if the analysis from OP is right) but its hard to rollback to a version which is not affected.