feat: add frontMatter.file.slugSeparator setting

Allows users to configure the word separator used when generating slugs
and file names from titles. Defaults to `-` (hyphen) to preserve existing
behavior. Set to `_` for underscore-separated file names.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

package.json

@@ -965,6 +965,12 @@
           "markdownDescription": "%setting.frontMatter.file.preserveCasing.markdownDescription%",
           "scope": "File"
         },
+        "frontMatter.file.slugSeparator": {
+          "type": "string",
+          "default": "-",
+          "markdownDescription": "%setting.frontMatter.file.slugSeparator.markdownDescription%",
+          "scope": "File"
+        },
         "frontMatter.framework.id": {
           "type": "string",
           "default": "",