RocDec::from_str accept 'e' exponent notation

The intent of this change is to allow roc dec values to be parsed from
strings using the 'e' exponent notation, eg '1e3' as a notation for '1000'.

Fixes #7351

crates/repl_test/src/tests.rs

@@ -40,6 +40,11 @@ fn literal_4point2() {
     expect_success("4.2", "4.2 : Frac *");
 }
 
+#[test]
+fn literal_4point2e3() {
+    expect_success("4.2e3", "4200 : Frac *");
+}
+
 #[test]
 fn num_addition() {
     expect_success("1 + 2", "3 : Num *");