refactor icon names to remove `_New` suffix

[ChitlangeSahas]

A lot of the Icon font names have _New as their suffix. We should refactor and remove that suffix.

[taramk]

We might as well improve some of the names at the same time—renaming BookCode to Notebook, for example. Some of the icons are using old names that don't quite match semantically. And be consistent as to whether icons are named for their purpose or for what they look like (ex. - Darkmode vs. Moon).