Allow text and regex show rules to match across styled content

Description

Currently, text and regex show rules don't match when using styled content, as in this example:

#show regex("an \w*"): underline

This is an example with regex show rules!

This is an _example_ with regex show rules!

It would be very useful if the show rule would match across styled content. To begin with, this could be limited to elements which have a clear text representation, such as emph in this case. For this, typst would build an internal plain-text representation of the sequence, joining all the element's plain-text representations (e.g. emph.body is text), and then match the regex against that text representation. Such an internal plain-text representation is already used to build PDF bookmarks.

For simplicity, text and regex should not match across linebreaks and parbreaks, i.e. the plain-text representation should start over new on every line- and parbreak. This would then also give ^ more meaning (see https://github.com/typst/typst/issues/5273)

For cases where multiple show rules match a piece of text disjoint, it will be necessary to split element functions. E.g. show "This is": underline together with show "is an": upper would require underline to be split, so as to apply upper, i.e. an equivalent to #underline[This ]#upper[#underline[is] an].

Use Case

Any use cases where text and regex show rules currently break or behave unexpectedly due to styled content. More specialized use cases include sentence casing (see https://github.com/alerque/decasify/issues/38), customization of bibliography (see this forum post).

Start with the text and regex show-rule implementation and the existing plain-text representation used to build PDF bookmarks. Investigate how styled sequences such as emph are represented, along with linebreak and parbreak boundaries. Done means matching can span supported styled content while preserving the described non-crossing and overlapping-rule behavior.