Eslint: Avoid relying on unsafe == and != js behavior

[s-rigaud]

Description

Add eslint rule to avoid relying on == or != that can lead to unexpected behavior. Most of the code is already using triple === or !==.

If this PR get approved I will create another one for == usage in addons and examples.

[github-actions]

📦 Bundle size

Full ESM build, minified and gzipped.

	Before	After	Diff
WebGL	339.2 79.01	339.21 79.01	+2 B +1 B
WebGPU	485.98 134.92	485.98 134.91	+4 B -3 B
WebGPU Nodes	485.44 134.82	485.45 134.82	+4 B -2 B

🌳 Bundle size after tree-shaking

Minimal build including a renderer, camera, empty scene, and dependencies.

	Before	After	Diff
WebGL	465.1 112.06	465.1 112.06	+2 B +0 B
WebGPU	555.38 150.4	555.38 150.4	+4 B -4 B
WebGPU Nodes	511.23 140.1	511.23 140.09	+3 B -2 B

[Mugen87]

I've reverted the rule via de85929 since it is too strict for the addons folder, see discussion in #30089.

[s-rigaud]

What do you think about keeping the rule but not applying it in addons for now ? This way the core source code can stay a bit safer and eslint does not complain for addons. The following eslint config does the trick:

  "overrides": [
      {
          "files": ["examples/jsm/**/*.js"],
          "rules": {
              "eqeqeq": "off"
          }
      }
  ]

[Mugen87]

IMO, we should not start to distinct rules per directories that makes everything unnecessarily complicated. Let's not over-engineer the ESLint usage and keeps things simple.