chore(contracts): switch from yarn to pnpm (#33)

Switch from yarn to pnpm for node package management.

#### Motivation

[pnpm](https://pnpm.io/) is becoming industry standard for managing node
packages. It's lightweight, and removes a lot of bloat from node_modules
- using symlinks and ensuring package versions are only installed to
disk once. Installations are much quicker. And it has better support for
monorepos (pnpm workspaces) should we decide to manage multiple pnpm
projects in our monorepo (as [optimism
does](https://github.com/ethereum-optimism/optimism/blob/develop/pnpm-workspace.yaml),
for example)

I've avoided using it in the past alongside foundry because foundry has
allegedly had [issues with handling pnpm
symlinks](foundry-rs/foundry#2714), but I've
tried it in our playground repo and imports work well, for both building
& IDE ls integrations.

task: none

.pre-commit-config.yaml

@@ -4,7 +4,7 @@ repos:
     rev: v4.5.0
     hooks:
       - id: trailing-whitespace # trims trailing whitespace
-        exclude: "(testdata|.yarn)"
+        exclude: "testdata"
       - id: end-of-file-fixer # ensures that a file is either empty, or ends with one newline
         exclude_types: ["proto"]
         exclude: "testdata"

contracts/.gitignore

@@ -1,13 +1,4 @@
-.yarn/*
-!.yarn/cache
-!.yarn/patches
-!.yarn/plugins
-!.yarn/releases
-!.yarn/sdks
-!.yarn/versions
-
 node_modules
-yarn-error.log
 
 out
 cache

contracts/package.json

@@ -1,5 +1,5 @@
 {
-  "packageManager": "yarn@4.0.2",
+  "packageManager": "pnpm@8.14.0",
   "dependencies": {
     "ds-test": "https://github.com/dapphub/ds-test",
     "forge-std": "https://github.com/foundry-rs/forge-std"