Make default PROJECT_FILES respect .gitattributes export-ignore

[TravisCarden]

The symlink-project command helpfully provides a way to override the list of files it should symlink via the PROJECT_FILES environment variable. This is great if you're willing to hardcode the list, but more than a little difficult to achieve if you want a dynamic list. In order to get the most production-like version of my module placed, I wanted to not symlink any files that would be omitted by the Drupal.org Composer Facade, i.e., those that aren't excluded via .gitattributes export-ignore. (At first I wanted to exclude those in .gitignore, too, but then I realized that could incorrectly exclude compiled code, like CSS and JS.) And I didn't want to hardcode that list and set future developers up for failure when they add new files and forget about it.

So I came up with a solution. It works for me, and I'm satisfied with it. I don't know if it will interest anyone else, but I thought I'd put it out there in case it could help someone. I don't necessarily expect that it will actually be merged.

How This PR Solves The Issue

It makes the default behavior in symlink-project exclude not only the hardcoded list of common files but also any that are in .gitattributes with export-ignore. (I also added "docroot" to the default list while I was there, since some hosting providers (e.g., Acquia) use that.

Manual Testing Instructions

ddev add-on get https://github.com/TravisCarden/ddev-drupal-contrib/tarball/feature/dynamic-project-files
ddev restart

# Add a file:
touch test.txt

# See that symlink-project symlinks it. (Expect output "test.txt")
ddev symlink-project 2>/dev/null | grep test.txt

# Add it to .gitattributes:
echo '/test.txt  export-ignore' | tee -a .gitattributes

# See that symlink-project does NOT symlinks it. (Expect no output.)
ddev symlink-project | grep test.txt

Automated Testing Overview

As I'm just "throwing this over the wall", I'm not taking the time to write automated tests.

Release/Deployment Notes

TBD