Add VRM expression, lookAt, and thumbnail metadata

[aaronfranke]

This PR adds several pieces of VRM metadata that I forgot about in #4. Namely:

• Expressions (VRM 1.0 only, since VRM 0.x expressions are hard-coded for blend shapes while Yinglet Creator uses texture transforms) (note that before this PR I already included expressions as animations, and Khronos character/avatar standard expressions, but I forgot about VRM's expressions).
• Look At angles (should prevent the pupil from yeeting off to wherever in compliant VRM apps).
• Thumbnail image.

Also, the generator/exporter strings have been updated to include the version number, like "Yinglet Creator 1.1.2".