Suppress built files in Gitlab MR via .gitattributes

Problem/Motivation

The bundle files that are built by svelte are not really readable, and they are not handled well by git diff, even Gitlab's version.
Apparently Gitlab has quietly adopted Github's approach to suppressing these, though it's not documented well

Steps to reproduce

View any MR that changes the build files, notice how they either never load or are illegible once they do

Proposed resolution

Add .gitattributes to hide them from Gitlab UI

Remaining tasks

• ✅ File an issue about this project
• ☐ Addition/Change/Update/Fix to this project
• ☐ Testing to ensure no regression
• ☐ Automated unit/functional testing coverage
• ☐ Developer Documentation support on feature change/addition
• ☐ User Guide Documentation support on feature change/addition
• ☐ Code review from 1 Drupal core team member
• ☐ Full testing and approval
• ☐ Credit contributors
• ☐ Review with the product owner
• ☐ Release

User interface changes

API changes

Data model changes

Release notes snippet