frontend: code styling: Add rules to enforce blank lines between blocks

[manuelwedler]

We would like to enforce stricter styling rules and add blank lines between function declarations and in general code blocks. This should be done in order to be consistent in the frontend codebase.

Just did a quick research. There are ESLint rules to achieve this:
https://eslint.org/docs/latest/rules/padded-blocks is probably the way to go.
We could also consider these two:
https://eslint.org/docs/latest/rules/padding-line-between-statements
https://eslint.org/docs/latest/rules/lines-between-class-members

Since applying these will change a lot of code, I would keep it out of the scope of this PR and just create an issue for now.

Originally posted by @manuelwedler in #2031 (comment)