feat!: rework text

[cakekindel]

This change brings the enum / contents pattern to the Text object, breaking its monolithic declaration into multiple mods and structs. This comes with a host of ergonomics improvements, like lots of handy conversions - for example if there is an fn of type fn foo(text: impl Into<text::Plain>) -> Foo, you can invoke it with a text::Plain, a string literal, or a String.

BREAKING!: text is now an enum of single-element tuples. Those tuples point to separate structure definitions for the different kinds of Text objects.

BREAKING!: text is now publicly used at the crate root, breaking glob imports like slack_blocks::* if there's a local code element named text