fix: Add a project-local .envrc file.

This fixes the issue described here:

#547 (comment)

It does so by configuring `direnv` to write its cache for this project
in `~/.cache/direnv/layouts`. This is a bit intrusive, but without
this workaround, `vite` commands search the local `.direnv` cache for
dependencies, and it adds about 10 seconds or more to every `vite`
command. Note that the fix comes from here:

https://github.com/direnv/direnv/wiki/Customizing-cache-location

Before falling back to this workaround, I also tried the following
Vite configuration tricks, but none of them worked:

https://vitejs.dev/config/server-options.html#server-watch
https://vitejs.dev/config/dep-optimization-options.html#optimizedeps-exclude

There are multiple (somewhat) related issues currently filed on Vite
which suggest that convincing Vite not to look everywhere in the
project root directory is a known issue:

https://github.com/vitejs/vite/pull/8778/files
vitejs/vite#7363
vitejs/vite#8341

.envrc

@@ -0,0 +1,13 @@
+# Prevent direnv from creating a local .direnv cache. Otherwise, 'vite'
+# commands will be incredibly slow to start.
+
+: ${XDG_CACHE_HOME:=$HOME/.cache}
+declare -A direnv_layout_dirs
+direnv_layout_dir() {
+    echo "${direnv_layout_dirs[$PWD]:=$(
+      local path="${PWD//[^a-zA-Z0-9]/-}"
+      echo "$XDG_CACHE_HOME/direnv/layouts/${path:1}"
+    )}"
+}
+
+use flake

.gitignore

@@ -13,7 +13,6 @@ result*
 
 # direnv.
 .direnv/
-.envrc
 
 # tsc
 tsconfig.tsbuildinfo

README.md

@@ -182,13 +182,7 @@ On the other hand, for production builds and CI, you can rest assured that TypeS
 
 We've tried to configure the project such that LSP and other tooling should "just work" in your editor, but you still might need to do some configuration on your end to hook it up. At the very least, the Nix development shell provides binaries for the TypeScript language server and `eslint` support, so you shouldn't need to download these or otherwise install them in your environment. (However, note to Emacs LSP users: it may prompt you whether it should automatically download a helper, and in my experience, this is usually necessary. I haven't gotten to the bottom of why this is required. -- @dhess)
 
-Probably the easiest way to ensure your editor sees the right environment is to add [`direnv`](https://direnv.net) support to your shell & editor, and then use [`nix-direnv`](https://github.com/nix-community/nix-direnv) to ensure that your editor sees the tools that the Nix shell provides. If you've got `nix-direnv` in your environment, you can do this by adding a file to the top-level project directory named `.envrc`, which contains just this one line:
-
-```
-use flake
-```
-
-If you need a tool that's not provided by the project's Nix development shell and you think others would benefit from it, as well, please make a PR!
+Probably the easiest way to ensure your editor sees the right environment is to add [`direnv`](https://direnv.net) support to your shell & editor, and then use [`nix-direnv`](https://github.com/nix-community/nix-direnv) to ensure that your editor sees the tools that the Nix shell provides. The project includes an `.envrc` file which will do the right thing once you run `direnv allow` in the project root directory.
 
 ### CI & Chromatic