Exclude src/node_modules from .gitignore.

[aickin]

src/node_modules is a directory of package stubs that is checked in for the test suite to use. Unfortunately, though, there is a node_modules line in .gitignore, which means the src/node_modules directory is ignored by git.

I ran into this in #10024, where I had to add a new file to src/node_modules, but because of the .gitignore, I failed to commit the new file. (Obviously, neither git commit -a nor git stage -A will add an ignored file.) My failure to commit the new file lead to a confusing situation where tests were working locally but not in CI, and it took me an hour or two to figure out what was going on.

This PR just excludes src/node_modules from .gitignore so that this kind of thing won't happen again.

Thanks for all your work!

[sophiebits]

thanks!