JavaScript - Webpack lesson: Adds filenames to code snippets that were missing

[ric-projects]

Because

Lesson clarity, adding missing filenames to existing code snippets, since we're working with multiple different files.

This PR

• Adds filenames to code snippets.

Issue

Closes #XXXXX

Additional Information

Lesson already had a few code snippets with filenames, but some were still missing. Added the missing ones for further clarity. Similar to merged PR https://github.com/TheOdinProject/curriculum/pull/29768

Pull Request Requirements

• [x] I have thoroughly read and understand The Odin Project curriculum contributing guide
• [x] The title of this PR follows the location of change: brief description of change format, e.g. Intro to HTML and CSS lesson: Fix link text
• [x] The Because section summarizes the reason for this PR
• [x] The This PR section has a bullet point list describing the changes in this PR
• [x] If this PR addresses an open issue, it is linked in the Issue section
• [x] If any lesson files are included in this PR, they have been previewed with the Markdown preview tool to ensure it is formatted correctly
• [x] If any lesson files are included in this PR, they follow the Layout Style Guide