A view's `icon` could only be a built-in icon name, so a plugin's tool
window button couldn't show the plugin's own mark. Like `fileIcons`, an
`icon` ending in `.svg` is now a path in the package: the host reads and
checks it at install time (a missing file or an SVG with `<script>` refuses
the install), serves it as the `viewIcon/<id>` asset, and the tool window
registers it as an external icon, falling back to the plugin icon if it
can't be loaded. Icon names work as before.
Closes#1335
Co-authored-by: yzPeedro <[email protected]>
Pedro Pessoa
7ffaa0b5da