Introduce openscad linter

[kellervater]

As community contributions get more frequent we need to align on a common way to write openscad code.
Therefore we should introduce an openscad linter.
When tackling this issue the assignee should:

• assess existing solutions which are viable
• decide on wheter to use such an existing solution or implement a custom linter
• implement/integrate the chosen solution

For now the only hard requirement is to format code in a common way via the linter.