'is_feed was called incorrectly' notice if `wp_styles()` is called before the query

[aaemnnosttv]

WordPress 5.8 introduced the wp_should_load_separate_core_block_assets (​https://developer.wordpress.org/reference/functions/wp_should_load_separate_core_block_assets/) function which calls is_feed() internally as part of its preliminary checks that guard the application of its filter.

Since wp_should_load_separate_core_block_assets is now called within wp_default_styles(), this has the potential to raise a "doing it wrong" notice if WP_Styles is initialized before the query runs for non-admin requests.

wp_styles() has never imposed such timing restrictions on when it is called, this is somewhat of a breaking change.

Ideally a different condition that does not impose the same limitation could be used in the place of is_feed here.

[sabernhardt]

function added in #50328