flake: Add meson formatter

This adds a meson.format file that mostly mirrors the projects
meson style and a pre-commit hook to enforce this style.
Some low-diff files are formatted.

tests/functional/plugins/meson.build

@@ -1,8 +1,6 @@
 libplugintest = shared_module(
   'plugintest',
   'plugintest.cc',
-  dependencies : [
-    dependency('nix-expr'),
-  ],
+  dependencies : [ dependency('nix-expr') ],
   build_by_default : false,
 )

tests/functional/test-libstoreconsumer/meson.build

@@ -1,8 +1,6 @@
 libstoreconsumer_tester = executable(
   'test-libstoreconsumer',
   'main.cc',
-  dependencies : [
-    dependency('nix-store'),
-  ],
+  dependencies : [ dependency('nix-store') ],
   build_by_default : false,
 )