Consider unreachable code to be unnecessary in VSC

This adds `unreachable_code` to the list of diagnostic codes we map to
`Unnecessary` in Visual Studio Code. This is consistent with what the
TypeScript language server does.

editors/code/src/utils/diagnostics/rust.ts

@@ -103,6 +103,7 @@ function isUnusedOrUnnecessary(rd: RustDiagnostic): boolean {
     return [
         'dead_code',
         'unknown_lints',
+        'unreachable_code',
         'unused_attributes',
         'unused_imports',
         'unused_macros',