Proper escaping for string literals

[netfri25]

Description

Applies unescaping for string literals in the lexer.
No things have been added to implement this, since it makes use of the unescape crate that was used in a different place.

Usage

"next line\ntabbed\tspace using hex code\x20"

Checklist

• All widgets I've added are correctly documented.
• I added my changes to CHANGELOG.md, if appropriate.
• The documentation in the docs/content/main directory has been adjusted to reflect my changes.
• I used cargo fmt to automatically format all code before committing

[netfri25]

this is also a fix for #84, but more general (every string literal instead of just label text)

[w-lfchen]

i feel like this pr should include switching the escaping crate, see #1194.
i might pick that pr up at some point with an actual implementation but i've lost the energy, time and motivation to work on eww as of writing.
just thought i should point this out because it seemed relevant

[netfri25]

this wasn't hard to implement, so I've just done it myself.
for now it doesn't really handle any errors, and when an error occures when escaping a string literal then it just returns the original unescaped string.