Use named export for esm compat/server

[marvinhagemeister]

• Ensure that we do the renderToString -> renderToStaticMarkup aliasing only here in compat/server
• Keep backwards compat for older versions of preact-render-to-string where the module object was a function. Preferred way is to use mod.renderToString.

This PR is in preparation for preactjs/preact-render-to-string#185

[coveralls]

Coverage remained the same at 99.445% when pulling 19885d3 on compat-server-export into c6ca49e on master.

[rahulbhadhoriya]

after this change i am getting this error while building

ERROR in ./node_modules/preact/compat/server.mjs
Module not found: Error: Can't resolve 'preact-render-to-string' in '/Users/B0216312/Desktop/node_modules/preact/compat'
@ ./node_modules/preact/compat/server.mjs 1:0-4:33 1:0-4:33 1:0-4:33
@ ./node_modules/google-maps-react/dist/components/InfoWindow.js
@ ./node_modules/google-maps-react/dist/index.js
@ ./app/modules/homes/pages/broadband/feasibility/Map.js
@ ./app/modules/homes/pages/broadband/feasibility/FeasibilitySmart.js
@ ./app/modules/homes/pages/broadband/feasibility/FeasibilityMain.js
@ ./app/modules/homes/pages/broadband/home-page/HomePage.js
@ ./app/modules/homes/lazy.js
@ ./app/modules/homes/routes.js
@ ./app/modules/homes/App.js
@ ./app/modules/homes/common/containers/Root.js
@ ./app/modules/homes/index.js
@ multi (webpack)-dev-server/client?http://0.0.0.0 (webpack)/hot/dev-server.js @babel/polyfill ./app/modules/homes/index.js