fix(plugin): declare esbuild so plugin install can compile TypeScript

[truffle-dev]

Description

opencli plugin create scaffolds a TypeScript plugin. Installing it with opencli plugin install file://<dir> runs transpilePluginTs (src/plugin.ts) to compile the source to .js via resolveEsbuildBin. That resolver only finds esbuild when it happens to be present — and in this repo esbuild is a purely transitive devDependency (it's in neither dependencies nor devDependencies; package-lock.json flags it "dev": true). So a production install (npm i -g @jackwener/opencli) ships no esbuild at all. The transpile then no-ops with a warning, no .js is produced, and discoverPluginDir (src/discovery.ts) aborts with no compiled .js found — breaking the documented create → install flow.

This declares esbuild as an optionalDependency so it installs alongside the CLI on supported platforms, while preserving the existing graceful degradation in resolveEsbuildBin / transpilePluginTs for platforms where the optional binary can't be installed. optionalDependencies is the idiomatic fit here (it mirrors how esbuild itself ships its platform binaries) and keeps the lean production-dependency footprint — esbuild is installed by default but never hard-fails the install.

Tradeoff for your call: if you'd rather guarantee the transpiler is always present, this could be a hard dependencies entry instead; optionalDependencies is the lighter promise that matches the code's already-graceful behaviour when esbuild is absent.

Related issue: Closes #1959

Type of Change

• 🐛 Bug fix
• ✨ New feature
• 🌐 New site adapter
• 📝 Documentation
• ♻️ Refactor
• 🔧 CI / build / tooling

Checklist

• I ran the checks relevant to this PR
• I updated tests or docs if needed
• I included output or screenshots when useful

Screenshots / Output

Added a regression test next to the existing resolveEsbuildBin test. The existing test passes today only because esbuild is present transitively in dev, which masks the production gap; the new test asserts esbuild is a declared production-installable dependency.

Red on main (no declaration):

 FAIL  src/plugin.test.ts > resolveEsbuildBin > declares esbuild as a production-installable dependency
   expect(declared).toBe(true)  ->  received false

Green with this change:

 Test Files  1 passed (1)
      Tests  73 passed (73)

npx tsc --noEmit exits 0. package.json and package-lock.json stay in sync (regenerated with npm install --package-lock-only), so npm ci is unaffected.