Rename all proposal plugins to -proposal- from -transform- (#6570)

packages/babel-plugin-proposal-function-sent/test/fixtures/function-sent/basic/exec.js

@@ -0,0 +1,13 @@
+let sent, yielded;
+
+function* gen() {
+    sent = function.sent;
+    yielded = yield;
+}
+
+const it = gen();
+it.next(1);
+it.next(2);
+
+assert.equal(sent, 1);
+assert.equal(yielded, 2);