[checks] Add check for missing whitespace in `if(`, `for(`, `while(`

[Eisenwave]

Related to #404

Following 4b07ece, we now consistently use:

• for (
• while (
• if (

always with a separating space in the whole document. While we usually give the editors some freedom when it comes to whitespace, it's worth being at least a bit consistent.

Anyhow, we ought to write some checks in check-source.sh which detect

• for(
• while(, or
• if(