Improve internal eslint and prettier checks
This commit in MR164 added trailing blanks to the project's .gitlab-ci.yml but this was not deteced in the "Check Code" job.
See comment !164 (comment 290128)
We have assets/internal/.eslintrc.js but the trailing blanks option is not specified. We are using Prettier due to having "plugin:prettier/recommended" as part of our eslint configuration, but we do not have any .prettierrc.json config file. Therefore some rules we enforce do not match the Core standards.
Proposed resolution
Add .prettierrc.json and match the Core formatting standards. The biggest change this introduces is to default to single quotes instead of double quotes. The Prettier rule is to reduce the number of escaped quotes in a string which is still enforced, but single quotes are now the prefered choice if everything else is equal.