Our eslintrc extends from 'wikimedia/client' which includes a
'no-restricted-properties' ruleset from the 'not-es5.js' file [1].
However, we were also including our own 'no-restricted-properties'
rules.
ESLint handle this duplication by clobbering instead of merging
so eslint-config-wikimedia's no-restricted-properties where not taking
effect and we were losing out on some guards against using es6.
This commit corrects that and makes both no-restricted-properties
rulesets merge instead of clobber as already done in MobileFrontend [2]
[1] 07320f16ae/language/not-es5.js (L5)
[2] https://github.com/wikimedia/mediawiki-extensions-MobileFrontend/blob/master/.eslintshared.js
Bug: T239269
Change-Id: Ibc2c144be51719d71a4c1d5828486253a5d4bf5d
