Release WASI 0.2.1 (#612)

* rename `preview2/` to `wasip2/`

* Update to v0.2.1

Author: yoshuawuyts

preview2/cli/command.wit

@@ -0,0 +1,10 @@
+package wasi:cli@0.2.1;
+
+@since(version = 0.2.0)
+world command {
+  @since(version = 0.2.0)
+  include imports;
+
+  @since(version = 0.2.0)
+  export run;
+}

preview2/cli/exit.wit

@@ -1,3 +1,4 @@
+@since(version = 0.2.0)
 interface environment {
   /// Get the POSIX-style environment variables.
   ///
@@ -7,12 +8,15 @@ interface environment {
   /// Morally, these are a value import, but until value imports are available
   /// in the component model, this import function should return the same
   /// values each time it is called.
+  @since(version = 0.2.0)
   get-environment: func() -> list<tuple<string, string>>;
 
   /// Get the POSIX-style arguments to the program.
+  @since(version = 0.2.0)
   get-arguments: func() -> list<string>;
 
   /// Return a path that programs should use as their initial current working
   /// directory, interpreting `.` as shorthand for this.
+  @since(version = 0.2.0)
   initial-cwd: func() -> option<string>;
 }