Prevent Windows console window for yaaknode and yaakprotoc

Add new_xplatform_command() helper in yaak-common that creates a
tokio::process::Command with CREATE_NO_WINDOW flag set on Windows.

Also converts git commands to async for consistency.

crates/yaak-common/Cargo.toml

@@ -6,3 +6,4 @@ publish = false
 
 [dependencies]
 serde_json = { workspace = true }
+tokio = { workspace = true, features = ["process"] }