Modify the a-an!an pattern to work for a-an!xn

The Kiss X4 is matching products with a "x4 zoom". The plan is to fix
this by treating the x4 classification as 'xn' instead of 'an'. This
would break the DSC-V100 / X100 alternate product code rule. So modify
it to work for !xn or !an.
Treat x4 pattern as 'xn' not 'an' to fix Kiss X4

The Kiss X4 was matching a product with a x4 optical zoom. To fix this
issue, treat a block with a single 'x' as having a classification of
'x'. Convert it back to a 'c' (consonant) if it is preceded by a dash
(hence a product code), or if it is not succeeded by a number.