fix: line endings on Windows

[empyrical]

Summary:

This pull request introduces a .gitattributes file into this repo to force git to use lf rather than crlf line endings on Windows. Without it, running yarn lint results in thousands of errors like this for Windows users:

   6:3   error  Expected linebreaks to be 'LF' but found 'CRLF'  linebreak-style
   6:3   error  Delete `␍`                                       prettier/prettier
   7:11  error  Expected linebreaks to be 'LF' but found 'CRLF'  linebreak-style
   7:11  error  Delete `␍`                                       prettier/prettier
   8:16  error  Expected linebreaks to be 'LF' but found 'CRLF'  linebreak-style
   8:16  error  Delete `␍`                                       prettier/prettier
   9:4   error  Expected linebreaks to be 'LF' but found 'CRLF'  linebreak-style
   9:4   error  Delete `␍`                                       prettier/prettier

This fix is the one suggested by the ESLint documentation:

https://eslint.org/docs/rules/linebreak-style#using-this-rule-with-version-control-systems

Test Plan:

Running yarn lint works properly on Windows with this fix in place.

[thymikee]

Thanks!