Support the selectors with `:is()` and `:where()`

[krassowski]

Problem

Selectors with comma are currently straight out rejected in a few places:

https://github.com/jupyterlab/lumino/blob/10d232e7d45572ed5fbb4389c3c9a4671ab0e5eb/packages/widgets/src/contextmenu.ts#L330-L333

https://github.com/jupyterlab/lumino/blob/10d232e7d45572ed5fbb4389c3c9a4671ab0e5eb/packages/commands/src/index.ts#L1561-L1566

This is understandable because the selector specificity can be only computed for individual selector, not a comma-separated list of selectors.

However, the :is() and :where() selectors now widely supported by browsers and take a comma-separated lists of selectors:

• :is() counts towards specificity with the specificity of the most specific element on the list
• :where() counts with specificity of zero (so very easy to implement!)

Lack of support for :is and :where makes creating selectors for certain commands and context menu more difficult than it should be in 2024.

Proposed Solution

Implement support for :is() and :where().

Additional context

Encountered this in https://github.com/jupyterlab/jupyterlab/pull/15845 when trying to make the context menu show for .jp-RunningSessions-item:is(.jp-mod-kernel,.jp-mod-kernel-widget).