web: Move root routes into a shared constant

Created by: xg-wang

Summary

This PR moves root routes into a shared constant. Sub routes are not (yet) moved to this top constant.

fixes https://github.com/sourcegraph/sourcegraph/issues/20693

Testing

Passed these:

yarn test
yarn all:eslint
yarn prettier-check
yarn all:stylelint
yarn graphql-lint

Note yarn integration-test fails, which also fails on main branch locally.