feat(fsmonitor): add .watchmanconfig to repo

This identifies the directory as Watchman-enabled. Additional config settings can go in this file. The contents of this file are based on https://github.com/thoughtpolice/buck2-nix/blob/fb8ed41881c481d4c876a8eb8c619cf25f3f077f/.watchmanconfig.

.watchmanconfig

@@ -0,0 +1,6 @@
+{
+  "enforce_root_files": true,
+  "root_files": [".watchmanconfig"],
+  "ignore_dirs": ["target"],
+  "ignore_vcs": [".git", ".sl", ".jj", ".hg"]
+}