gh-112571: Move fish venv activation script into the common folder (GH-117169)

pythongh-112571: allow using fish venv activation script on windows

The fish shell can be used on windows under cygwin or msys2.
This change moves the script to the common folder
so the venv module will install it on both posix and nt systems (like the bash script).

diff --git a/Lib/venv/scripts/posix/activate.fish b/Lib/venv/scripts/common/activate.fish
similarity index 100%
rename from Lib/venv/scripts/posix/activate.fish
rename to Lib/venv/scripts/common/activate.fish