Eslint camelCase (#1625)

* Add eslint rule to flag up me forgetting to camelCase - I'd say I introduced all the snake_cases that are fixed here
* Allow some dubious snake cases for now; we could examine again whether they can be converted to `camelCase['snake_var']` if we need to maintain backwards compatibility of output
* add ESLINT_USE_FLAT_CONFIG against eslint v8.57.0 in order to continue using the deprecated .eslintrc.js method

.eslintrc.js

@@ -23,5 +23,8 @@ module.exports = {
   rules: {
     'tsdoc/syntax': 'warn',
     '@typescript-eslint/prefer-as-const': 'warn',
+    'camelcase': ['error', {
+      allow: ['rr_.*', 'legacy_.*', 'UNSAFE_.*', '__rrweb_.*'],
+    }],
   },
 };