Use heck instead of convert_case for nu-derive-value (#13708)

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# Description
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@sholderbach mentioned that I introduced `convert_case` as a dependency
while we already had `heck` for case conversion. So in this PR replaced
the use `convert_case` with `heck`. Mostly I rebuilt the `convert_case`
API with `heck` to work with it as I like the API of `convert_case` more
than `heck`.

# User-Facing Changes
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Nothing changed, the use of `convert_case` wasn't exposed anywhere and
all case conversions are still available.

# Tests + Formatting
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No new tests required but my tests in `test_derive` captured some errors
I made while developing this change, (hurray, tests work 🎉)
- 🟢 `toolkit fmt`
- 🟢 `toolkit clippy`
- 🟢 `toolkit test`
- 🟢 `toolkit test stdlib`

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crates/nu-derive-value/src/from.rs

@@ -1,4 +1,3 @@
-use convert_case::{Case, Casing};
 use proc_macro2::TokenStream as TokenStream2;
 use quote::{quote, ToTokens};
 use syn::{
@@ -6,7 +5,10 @@ use syn::{
     Type,
 };
 
-use crate::attributes::{self, ContainerAttributes};
+use crate::{
+    attributes::{self, ContainerAttributes},
+    case::{Case, Casing},
+};
 
 #[derive(Debug)]
 pub struct FromValue;
@@ -393,7 +395,7 @@ fn derive_enum_from_value(
 /// all possible variants.
 /// The input value is expected to be a `Value::String` containing the name of the variant formatted
 /// as defined by the `#[nu_value(rename_all = "...")]` attribute.
-/// If no attribute is given, [`convert_case::Case::Snake`] is expected.
+/// If no attribute is given, [`snake_case`](heck::ToSnakeCase) is expected.
 ///
 /// If no matching variant is found, `ShellError::CantConvert` is returned.
 ///