feat(inspector): Add preview for templates in the inspector

[abstrakt8]

What has changed?

• Fixes Previewing Templates #1465

TemplatePreview.mp4

TemplatePreview.reactive.mp4

Implementation details

• Takes into account that entities that are not enabled in the hierarchy are also not rendered.
• Watching for material changes (because they are not part of the template)
• Watching for template changes (had to do an engine fix)
• Display a text "No preview available" if no mesh instances were found.

Dependencies

• Engine release with fix(template-asset): invalidate cached _previewRoot upon receiving new data engine#8003

Not in scope

• Template thumbnails (Template Thumbnails #783) due to uncertanity of how it would run. However, you can help me test by setting this line to true:

  editor/src/common/pcui/element/element-asset-thumbnail.ts

  Line 29 in 793d204

  'template': false // TODO: Change to true after testing with optimisations (GH Issue #783)

• I confirm I have read the contributing guidelines

[Maksims]

One significant think to look at, is that shader compilation is inevitable in this case. And it can be of serious problem in projects with more complex setups and combinations of materials.
We already have a problem where navigating to a folder with 10 materials, freezes browser for 5+ seconds.

To avoid that, we would need to ensure not too many thumbnails are ever attempted to be rendered in a single frame.

[willeastcott]

@abstrakt8 With Engine 2.12.x released, I guess this can be merged now?

[abstrakt8]

Still looks good after merging in from main

[kpal81xd]

Approving with couple of comments