Change matrix-sdk-ffi to rely on features over platform targets

The system of platform targets was already quite messy, and becoming
even worse as we start preparing for Wasm support. Switch to features
instead to make this easier to work with.

Author: Daniel Salinas

Cargo.lock

@@ -1,6 +1,6 @@
 [package]
 name = "matrix-sdk-ffi"
-version = "0.12.0"
+version = "0.11.0"
 edition = "2021"
 homepage = "https://github.com/matrix-org/matrix-rust-sdk"
 keywords = ["matrix", "chat", "messaging", "ffi"]