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

[krassowski]

Problem

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

lumino/packages/widgets/src/contextmenu.ts

Lines 330 to 333 in 10d232e

	function validateSelector(selector: string): string {
	if (selector.indexOf(',') !== -1) {
	throw new Error(`Selector cannot contain commas: ${selector}`);
	}

lumino/packages/commands/src/index.ts

Lines 1561 to 1566 in 10d232e

	function validateSelector(
	options: CommandRegistry.IKeyBindingOptions
	): string {
	if (options.selector.indexOf(',') !== -1) {
	throw new Error(`Selector cannot contain commas: ${options.selector}`);
	}

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 jupyterlab/jupyterlab#15845 when trying to make the context menu show for .jp-RunningSessions-item:is(.jp-mod-kernel,.jp-mod-kernel-widget).