Make <LitStr as FromMeta>::from_string no longer return an error

[Adhalianna]

The FromMeta implementation for syn::LitStr is now more complete fixing difficult to debug macro
panics that would occur when the type was used to build the macro.

Without this commit I would get the following panics:

error: proc-macro derive panicked
 --> models/src/lib.rs:1:44
  |
1 | #[derive(serde::Deserialize, Debug, Clone, jayapi::AsResource)]
  |                                            ^^^^^^^^^^^^^^^^^^
  |
  = help: message: called `Result::unwrap()` on an `Err` value: Error { kind: UnexpectedType("string"), locations: [], span: None }

for my derive macro built using darling. One of the structs nested on the one that had derived darling::FromDeriveInput used syn::LitStr type as its field. With this commit which actually adds only from_string function, lines 579-581, (the rest is copy-paste from the from_meta_lit! macro) the panic is gone.

I am not familiar with syn so I guess there might be a smarter way of creating LitStr from a &str but this works for me perfectly fine.

[TedDriggs]

How/why are you getting into the situation that this is being called with a &str while the receiver field wants a LitStr?

I'm torn on this, because the point of using a LitStr receiver field rather than a String would be to keep the span information, so we're adding a new impl in order to allow someone to wrap a String without that span information.