Support Custom Media Queries

When designing documents that use media queries, the same media query may be used in multiple places, such as to qualify multiple [@import](https://www.w3.org/TR/css-cascade-5/#at-ruledef-import) statements. Repeating the same media query multiple times is an editing hazard; an author making a change must edit every copy in the same way, or suffer from difficult-to-find bugs in their CSS.

(_From https://drafts.csswg.org/mediaqueries-5/#custom-mq_)

This is currently supported by Lightning CSS.

Read the CSS Media Queries Level 5 custom media query section and compare the behavior supported by Lightning CSS. Trace esbuild's CSS processing entry points, then verify that named custom media queries can be reused and expanded correctly, including in repeated @import media conditions.

Written by the indexing model from the issue text.