fix(stylize)!: add Stylize impl for String

[joshka]

Although the Stylize trait is already implemented for &str which
extends to String, it is not implemented for String itself. This
commit adds an impl of Stylize that returns a Span<'static> for String
so that code can call Stylize methods on temporary Strings.

E.g. the following now compiles instead of failing with a compile error
about referencing a temporary value:

let s = format!("hello {name}!", "world").red();

BREAKING CHANGE: This may break some code that expects to call Stylize
methods on String values and then use the String value later. This
will now fail to compile because the String is consumed by set_style
instead of a slice being created and consumed.

This can be fixed by cloning the String. E.g.:

let s = String::from("hello world");
let line = Line::from(vec![s.red(), s.green()]); // fails to compile
let line = Line::from(vec![s.clone().red(), s.green()]); // works

Fixes https://discord.com/channels/1070692720437383208/1072907135664529508/1148229700821450833

[codecov]

Codecov Report

Merging #466 (f94cec4) into main (343c6cd) will increase coverage by 0.09%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #466      +/-   ##
==========================================
+ Coverage   90.00%   90.10%   +0.09%     
==========================================
  Files          40       40              
  Lines       11156    11267     +111     
==========================================
+ Hits        10041    10152     +111     
  Misses       1115     1115              

Files Changed	Coverage Δ
src/style/stylize.rs	100.00% <100.00%> (ø)