Slots with comments as first element halts builds

[jakobrosenberg]

To Reproduce

<slot>foobar<!-- placeholder--></slot> works
<slot> <!-- placeholder--></slot> works
<slot></slot> works
<slot/> works
<slot><!-- placeholder--></slot> doesn't work
<slot><!-- placeholder--> </slot> doesn't work
<slot><!-- placeholder--> foobar </slot> doesn't work

Expected behavior
Should build

Svelte version
3.20.0 and 3.20.1

Rollup

Severity
Severe. Breaks apps and shows nothing in the logs.

[Conduitry]

The fix for this has been finally released in 3.21.0.