Fix .gitignore and add .editorconfig

• *build*/ is not needed because Meson puts a .gitignore in builddir already.

• meson/ why?

• /gst* is too wide, it includes e.g. gst-env.py, list symlinks explicitly instead.

• subprojects/* excludes all .wrap files, and !subprojects/gst* is not enough because e.g. macos-bison-binary is still ignored. Instead ignore only directories except those we include in our git repository.

• Add .editorconfig with some common settings to help most IDEs, could be extended for specific file formats in the future.