Support annotations, like those in Chrome

Would be good to support both import for these, and to allow or their creation in speedscope.

This would be a fairly significant change spanning import, UI, & file format

Start by mapping the profile import path, the annotation-related UI entry points, and the profile file-format handling. Compare the Chrome annotation data shown in the issue with the formats speedscope currently accepts. Done means annotations can be imported and created in the UI, with the file format preserving them.

Written by the indexing model from the issue text.