Add /*@__PURE__*/ annotation for better tree-shaking support (#470)

Hi,

We are using `protoc-gen-es` to generate TypeScript files for our
protobuf messages. In our project, there are unused messages that get
compiled into JavaScript files but are not imported anywhere. Since
we're using Vite, which relies on Rollup for bundling, Rollup does not
remove makeMessageType function calls from the output bundle, as seen in
the example below:

```js
f.makeMessageType("blacklist.SearchUnblacklistsByPhonesResponse", () => [{
  no: 1,
  name: "unblacklisted_phones",
  kind: "scalar",
  T: 9,
  repeated: !0
}]);
```

Rollup assumes that `f.makeMessageType` may have side effects (such as
mutating the global state), leading to unnecessary code being included
in the final bundle. As a result, our bundle size has doubled, becoming
significantly larger.

To address this issue, I've added the `/*@__PURE__*/` annotation before
`makeMessageType` function calls. This informs Rollup that these
function calls do not have any side effects, as documented in the
[Rollup configuration
options](https://rollupjs.org/configuration-options/#treeshake-annotations).
Adding this annotation will allow Rollup to better optimize the output
bundle by eliminating unused code.

If you have any suggestions for a better solution or improvements,
please leave a comment.

Thank you!