JavaScript style checks in code editor

The editor now checks for common problems in JavaScript code, like

• duplicate or conflicting var, function and formal parameter declarations
• use of vars and functions before their declaration
• == or != with 0, null, true, false, undefined and "
• comma expressions (outside of for statements)
• expression statements (unless calls, assignments or delete)
• assignments in a condition (like if (a = b))
• case blocks in a switch that don't end with return, break, continue or throw and are not empty
• nested block statements
• with statements
• void expressions