Add hidden `.briefcase` directory to `.gitignore` #251

Job	Run time
Pre-commit checks / Pre-commit Checks	29s
Unit tests	35s
Build app (ppb, macos-latest) / Verify App Build	3m 35s
Build app (ppb, ubuntu-22.04) / Verify App Build	7m 27s
Build app (pygame, macos-latest) / Verify App Build	4m 3s
Build app (ppb, windows-latest) / Verify App Build	3m 12s
Build app (pygame, ubuntu-22.04) / Verify App Build	9m 56s
Build app (pygame, windows-latest) / Verify App Build	3m 14s
Build app (pyside6, macos-latest) / Verify App Build	6m 10s
Build app (pyside6, ubuntu-22.04) / Verify App Build	16m 28s
Build app (pyside6, windows-latest) / Verify App Build	4m 10s
Build app (toga, macos-latest) / Verify App Build	13m 43s
Build app (toga, windows-latest) / Verify App Build	7m 14s
Build app (toga, ubuntu-22.04) / Verify App Build	13m 17s
Verify project (ppb, macos-latest) / Verify Project	1m 17s
Verify project (ppb, ubuntu-22.04) / Verify Project	55s
Verify project (ppb, windows-latest) / Verify Project	1m 40s
Verify project (pygame, macos-latest) / Verify Project	1m 28s
Verify project (pygame, ubuntu-22.04) / Verify Project	46s
Verify project (pygame, windows-latest) / Verify Project	1m 55s
Verify project (pyside6, macos-latest) / Verify Project	1m 13s
Verify project (pyside6, ubuntu-22.04) / Verify Project	47s
Verify project (pyside6, windows-latest) / Verify Project	1m 54s
Verify project (toga, macos-latest) / Verify Project	1m 16s
Verify project (toga, ubuntu-22.04) / Verify Project	54s
Verify project (toga, windows-latest) / Verify Project	1m 59s
	1h 49m 37s

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