feat(negative_spelling): create new exam exercise for rust

subjects/negative_spelling/README.md

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+## negative_spelling
+
+### Instructions
+
+In this exercise, you'll create the function `neg_spell` that will spell a negative number.
+
+Here are some examples of what your function should return:
+
+- `-1` -> `"minus one"`
+- `-14` -> `"minus fourteen"`
+- `-348` -> `"minus three hundred forty-eight"`
+- `-1002` -> `"minus one thousand two"`
+
+> Only negative numbers will be accepted, up to `"minus one million"`.
+> If a positive number is passed the function should return `"error: positive number"`.
+
+### Expected function
+
+```rust
+pub fn neg_spell(n: i64) -> String {
+
+}
+```
+
+### Usage
+
+Here is a program to test your function.
+
+```rust
+use spelling::*;
+
+fn main() {
+    println!("{}", neg_spell(-1234));
+    println!("{}", neg_spell(100));
+}
+```
+
+And its output:
+
+```console
+$ cargo run
+minus one thousand two hundred thirty-four
+error: positive number
+$
+```
+
+### Notions
+
+- [patterns](https://doc.rust-lang.org/book/ch18-00-patterns.html)