Severity options in custom rules should not have quotes

[hlprasu]

Severity options should not have quotes. Otherwise, it will be a syntax error in DRCs.

Example:

(rule "A"
  (constraint hole_clearance (min "0.5mm"))
  (severity exclusion))
             ^^^^^^^^ <- correct

(rule "A"
  (constraint hole_clearance (min "0.5mm"))
  (severity "exclusion"))
             ^^^^^^^^^ <- syntax error here

[hlprasu]

@mvnmgrx Kindly review and merge if acceptable.