style(lib/core): var -> let and const codemod (#4457)

Just like #4451 but for
`lib/core`

This does add an `.eslintignore` line, perhaps we'd rather skip a file a
different way.

Smaller part of the full vision:
#4444

Author: gaiety-deque

.eslintignore

@@ -0,0 +1,12 @@
+**/node_modules/*
+**/tmp/*
+
+build/tasks/aria-supported.js
+
+doc/api/*
+doc/examples/jest_react/*.js
+
+lib/core/imports/*.js
+lib/core/utils/uuid.js
+axe.js
+axe.min.js

eslint.config.js

@@ -189,6 +189,7 @@ module.exports = [
       'doc/api/*',
       'doc/examples/jest_react/*.js',
       'lib/core/imports/*.js',
+      'lib/core/utils/uuid.js',
       'axe.js',
       'axe.min.js'
     ]