Mark MappingTarget as #[non_exhaustive] for increased semver flexibility

This will allow us to add new enum variants in the future without breaking
semver compatibility. See
https://doc.rust-lang.org/reference/attributes/type_system.html#the-non_exhaustive-attribute

Since we already added an enum variant since v0.18.3, now is a good time to mark
it as `#[non_exhaustive]`.

src/syntax_mapping.rs

@@ -8,6 +8,7 @@ use globset::{Candidate, GlobBuilder, GlobMatcher};
 pub mod ignored_suffixes;
 
 #[derive(Debug, Clone, Copy, PartialEq)]
+#[non_exhaustive]
 pub enum MappingTarget<'a> {
     /// For mapping a path to a specific syntax.
     MapTo(&'a str),