wmllint: Implement [terrain_type] aliasof conversion for changes in 1.11.[89]

Adds a text transform step for the aliasof attribute under
[terrain_type] accounting for several changes to base terrain aliasing
in Wesnoth 1.11.8 and 1.11.9, including:

 * 10854d4802c4953cf05a8e28cb81ba5ab8d52b29 and related commits
 * c25849b7ea0a18e32d9065cfca91e703e179cf04 (Vit -> Vt in 1.11.9)

Just like usual terrain string conversions, this step can be disabled
for a specific line using # wmllint: noconvert.