build: Bump version 0.13.0 and API version to 0.13.

meson.build

-project('libinputsynth', 'c', version: '0.12.1',
+project('libinputsynth', 'c', version: '0.13.0',
   meson_version: '>= 0.45.1',
 )
 

src/meson.build

 so_version = 0
-api_version = '0.12'
+api_version = '0.13'
 api_path = 'libinputsynth-' + api_version
 
 version_split = meson.project_version().split('.')