Add .gitattributes to normalize how line endings are stored

[liamdon]

Related to issue #4238

Problem

Line endings in the repo are inconsistent between CRLF (Windows) and LF (Unix/Git).

Solution

Add a .gitattributes file so that git will normalize files as they are checked in, while still allowing developers to use their native line endings locally.

Issue #4238 suggests an additional cleanup task to fix existing files that were already checked in as CRLF. I am happy to open that task as a PR as well (just let me know), but I thought that the core maintainers might prefer to do so since it will be a scary diff (every line changed in 30 files).

Context

Currently around 5% of the source files are stored as CRLF (Windows), and the other 95% are LF (Unix) (see full report in #4238).

This causes issues for developers on both platforms:

• It can be tricky to ensure your editor retains the existing line endings for a given file, but if you change a file's line endings the resulting diff will show every line as changed. This makes it hard to review the PR! (example)
• Developers might also be confused about whether they should be changing line endings deliberately for consistency.
  For example, mat4.js and vec2.js use different line endings - should a developer change them to be consistent, or leave them as they are? What format should they use for a new file?

By using .gitattributes to normalize line endings at checkin/checkout, no-one will have to think about line endings ever again 🥳

I confirm I have read the contributing guidelines and signed the Contributor License Agreement.

[willeastcott]

This is great @liamdon - thanks so much. And yep, it would be a big help if you also attended to the 5% of files which use CRLF. 🙏