JavaScript folder: rename few folders/files for consistency

[01zulfi]

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• I have thoroughly read and understand The Odin Project Contributing Guide
• 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

Complete the following checkboxes ONLY IF they are applicable to your PR. You can complete them later if they are not currently applicable:

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1. Because:

Related to:

• Cleaning up folder/file structure in the curriculum repository top-meta#109

2. This PR:

1. Renames files/folders to snake_case
2. Renames files/folders to match course structure on the website

3. Additional Information:

Currently, this pull request addresses the following folders:

1. introduction
2. organizing-js
3. finishing-up
4. js-and-the-backend
5. js-in-the-real-world

A future pull request will address the rest of the folders in javascript.

[zachmmeyer]

Just have some conflicts to resolve