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-protocol/src/value/from_value.rs

@@ -22,7 +22,7 @@ use std::{
 /// the struct maps to a corresponding field in the record.
 /// You can customize the case conversion of these field names by using
 /// `#[nu_value(rename_all = "...")]` on the struct.
-/// Supported case conversions include those provided by [`convert_case::Case`], such as
+/// Supported case conversions include those provided by [`heck`], such as
 /// "snake_case", "kebab-case", "PascalCase", and others.
 /// Additionally, all values accepted by
 /// [`#[serde(rename_all = "...")]`](https://serde.rs/container-attrs.html#rename_all) are valid here.
@@ -35,7 +35,7 @@ use std::{
 ///
 /// Only enums with no fields may derive this trait.
 /// The expected value representation will be the name of the variant as a [`Value::String`].
-/// By default, variant names will be expected in ["snake_case"](convert_case::Case::Snake).
+/// By default, variant names will be expected in ["snake_case"](heck::ToSnakeCase).
 /// You can customize the case conversion using `#[nu_value(rename_all = "kebab-case")]` on the enum.
 ///
 /// Additionally, you can use `#[nu_value(type_name = "...")]` in the derive macro to set a custom type name

crates/nu-protocol/src/value/into_value.rs

@@ -12,8 +12,8 @@ use crate::{Record, ShellError, Span, Value};
 /// [`Value::Record`], where each field of the record corresponds to a field of the struct.
 /// By default, field names will be kept as-is, but you can customize the case conversion
 /// for all fields in a struct by using `#[nu_value(rename_all = "kebab-case")]` on the struct.
-/// All useful case options from [`convert_case::Case`] are supported, as well as the
-/// values allowed by [`#[serde(rename_all)]`](https://serde.rs/container-attrs.html#rename_all).
+/// All case options from [`heck`] are supported, as well as the values allowed by
+/// [`#[serde(rename_all)]`](https://serde.rs/container-attrs.html#rename_all).
 ///
 /// For structs with unnamed fields, the value representation will be [`Value::List`], with all
 /// fields inserted into a list.
@@ -21,7 +21,7 @@ use crate::{Record, ShellError, Span, Value};
 ///
 /// Only enums with no fields may derive this trait.
 /// The resulting value representation will be the name of the variant as a [`Value::String`].
-/// By default, variant names will be converted to ["snake_case"](convert_case::Case::Snake).
+/// By default, variant names will be converted to ["snake_case"](heck::ToSnakeCase).
 /// You can customize the case conversion using `#[nu_value(rename_all = "kebab-case")]` on the enum.
 ///
 /// ```