Prune stale gitignore rules

After a few changes to our build system, lots of gitignore rules
are obsolete. Meanwhile, in-tree CMake builds are missing some rules
like "/cargo/".

Drop the obsolete ones, and add the in-tree CMake ones for now.
Also add ".venv/" (used by build_tools/release.sh).
Also limit some rules like .vscode to top-level (?).

.gitignore

@@ -20,7 +20,6 @@
 *.o
 *.obj
 *.orig
-!tests/*.out
 *.out
 *.pch
 *.slo
@@ -36,46 +35,31 @@
 Desktop.ini
 Thumbs.db
 ehthumbs.db
+__pycache__/
 
 .directory
 .fuse_hidden*
 
 
-# Directories that only contain transitory files from building and testing.
-/doc/
-/share/man/
-/share/doc/
-/test/
-/user_doc/
-
-# File names that can appear in the project root that represent artifacts from
-# building and testing.
-/FISH-BUILD-VERSION-FILE
-/command_list.txt
-/command_list_toc.txt
-/compile_commands.json
-/doc.h
+# Artifacts from in-tree builds ("cmake .").
+/build.ninja
+/cargo/
+/CMakeCache.txt
+/CMakeFiles/
+/cmake_install.cmake
 /fish
-/fish.pc
 /fish_indent
 /fish_key_reader
-/fish_tests
-/lexicon.txt
-/lexicon_filter
-/toc.txt
-/version
-fish-build-version-witness.txt
-__pycache__
+/fish-localization-map-cache/
+/fish.pc
+/fish.pc.noversion
+/.ninja_log
 
 # File names that can appear below the project root that represent artifacts
 # from building and testing.
-/doc_src/commands.hdr
-/doc_src/index.hdr
-/po/*.gmo
 /share/__fish_build_paths.fish
-/share/pkgconfig
-/tests/*.tmp.*
 /tests/.last-check-all-files
+/.venv/
 
 # xcode
 ## Build generated
@@ -83,24 +67,19 @@ __pycache__
 *.xccheckout
 *.xcscmblueprin
 .vscode
-/DerivedData/
 /build/
+/DerivedData/
 /tags
-xcuserdata/
+/xcuserdata/
 
 # Generated by Cargo
 # will have compiled files and executables
-debug/
-target/
-
+/target/
 # These are backup files generated by rustfmt
 **/*.rs.bk
 
-# MSVC Windows builds of rustc generate these, which store debugging information
-*.pdb
-
 # Generated by clangd
-/.cache
+/.cache/
 
 # JetBrains editors.
 .idea/