Split "v2" branch into individual features

[VojtechVitek]

Split "v2" branch

• https://github.com/webrpc/webrpc/tree/schema-v2
• v0.8 with schema updates #134
• schema v2, aka webrpc-gen v0.8.0 #123

into individual features:

• Internal renaming
  • Internal renaming #168
• Breaking RIDL changes (message->type)
  • v0.9.0: Rename RIDL "message" to "struct", schema "messages" to "types" #171
• Breaking schema changes ("messages"->"types" etc.)
  • v0.9.0: Rename RIDL "message" to "struct", schema "messages" to "types" #171
• Alias type
  • Alias type #175
• Custom errors
  • Custom errors: Define API errors in RIDL #72
• Flatten request/response arguments (short-hand RIDL syntax)
  • Flatten request/response arguments (short-hand RIDL syntax) #183

Note: We have decided not to bump schema to "v2" for now, since we're still in the unstable v0.x.x release cycle. The goal is to reach stable webrpc@v1.0.0. We will provide a migration guide for any breaking changes we'll introduce along the way.