GH-177: ensure symlink-project adds /vendor/ and /web/ to gitignore

[tormi]

The Issue

• Ensure symlink-project adds /vendor/ and /web/ to .gitignore #177

After running the documented Drupal contrib setup flow, .gitignore is not updated automatically with /vendor/ and /web/, which can lead to accidental commits of generated scaffold directories.

How This PR Solves The Issue

• Updates ddev symlink-project to ensure .gitignore contains:
  • /vendor/
  • /web/
• Keeps the behavior idempotent by appending each entry only when it is missing.
• Preserves existing .gitignore content.

Manual Testing Instructions

ddev add-on get https://github.com/tormi/ddev-drupal-contrib/tarball/GH-177
ddev restart

Then verify:

• Run ddev symlink-project
• Confirm .gitignore includes /vendor/ and /web/
• Run ddev symlink-project again and confirm no duplicate entries are added

Automated Testing Overview

• Added a Bats test in tests/full.bats:
  • @test "ddev symlink-project updates gitignore"
• The test validates both entries are present and confirms idempotency (single occurrence after repeated runs).
• In this local session, full Bats execution was not completed because ./tests/bats/bin/bats was not available until submodules are initialized.

Release/Deployment Notes

• No deployment changes required.
• Change is limited to add-on command behavior and test coverage.
• Backward compatible and safe for existing projects because updates are append-only when entries are absent.

[weitzman]

OK, I'll take this despite my reply in #25. Could you rework it so that we do nothing if .gitignore already exists? Lets assume the project knows what it is doing in that case.