Add block scalar newline warning (#295)

In YAML, block scalars (`|` and `>`) silently add a trailing newline by default
("clip" chomping). This can cause subtle bugs when the newline is unintentional.

This PR adds a warning when clip chomping is used in fields where trailing
newlines commonly cause issues:

- Environment variables (workflow, job, step, container, service levels)
- Action inputs (`with:`)
- Reusable workflow inputs and secrets
- Job outputs
- Matrix values (including `include` and `exclude`)
- Concurrency groups

The warning suggests using `|-` (strip) or `|+` (keep) to be explicit.

Intentionally does NOT warn for:
- `run:` scripts (trailing newlines are normal)
- Fields trimmed server-side (`if:`, `name:`, `runs-on:`, etc.)

The feature is gated behind the `blockScalarChompingWarning` feature flag.

languageserver/src/connection.ts

@@ -123,7 +123,8 @@ export function initConnection(connection: Connection) {
       actionsMetadataProvider: getActionsMetadataProvider(client, cache),
       fileProvider: getFileProvider(client, cache, repoContext?.workspaceUri, async path => {
         return await connection.sendRequest(Requests.ReadFile, {path} satisfies ReadFileRequest);
-      })
+      }),
+      featureFlags
     };
 
     const result = await validate(textDocument, config);