file_icons: Use a separate icon key for Markdown files (#24648)

This PR updates the file icon mappings such that Markdown (`.md`,
`.markdown`) files map to the `markdown` key.

Release Notes:

- Icon themes: Added the ability to change the file icon for Markdown
(`.md`, `.markdown`) files.

crates/theme/src/icon_theme.rs

@@ -94,6 +94,7 @@ const FILE_ICONS: &[(&str, &str)] = &[
     ("lock", "icons/file_icons/lock.svg"),
     ("log", "icons/file_icons/info.svg"),
     ("lua", "icons/file_icons/lua.svg"),
+    ("markdown", "icons/file_icons/book.svg"),
     ("metal", "icons/file_icons/metal.svg"),
     ("nim", "icons/file_icons/nim.svg"),
     ("nix", "icons/file_icons/nix.svg"),