Update createPage to support windows paths (#243

Paths like `C:\Users\username\Documents\Github\studio-prototype\packages\studio\tests\__mocks__/test.tsx` are not considered absolute by `path.isAbsolute` in the frontend. This is because we are using `path-browserify` (a browser version of nodejs's `path`) which does not have proper windows support. In real nodejs, `path.isAbsolute` will return true for a windows path like the above.
browserify/path-browserify#1

We could look for an alternative but I'm not sure if that will be necessary. I think I'll have a better picture of that when I'm further in with getting the unit tests compatibility.

For the createPage action, the path.isAbsolute check is unnecessary since we do a `filepath.startsWith(pagesPath)` check as well, and `pagesPath` is always absolute. We may want to support a relative pagesPath at which point we may need to add additional validation, but it would probably make more sense for that validation to go into the backend.

J=SLAP-2791
TEST=auto

tests for createPage.ts and AddPageButton are now passing

packages/studio/src/store/StudioActions.ts

@@ -278,7 +278,7 @@ export default class StudioActions {
     }
     const pagesPath = this.getStudioConfig().paths.pages;
     const filepath = path.join(pagesPath, pageName + ".tsx");
-    if (!path.isAbsolute(filepath) || !filepath.startsWith(pagesPath)) {
+    if (!filepath.startsWith(pagesPath)) {
       throw new Error(`Error adding page: pageName is invalid: ${pageName}`);
     }
     const errorChars = pageName.match(/[\\/?%*:|"<>]/g);