Formatting: Extend `wp_kses_hair` and `wp_kses_hair_parse` to allow digits and underscores.

Fixes a lot of issues around parsing XML/HTML attributes.

Fixes #49464.

See #34406, #48608.

Props codeforest, zodiac1978, johnpgreen, dlh, ayeshrajans, johnpgreen, rilwis, travisnorthcutt, miqrogroove, chriscct7, whyisjake.

Built from https://develop.svn.wordpress.org/trunk@48132


git-svn-id: https://core.svn.wordpress.org/trunk@47901 1a063a9b-81f0-0310-95a4-ce76da25c4cd