feat: upgrade esbuild to ^0.15.0

[mrgrain]

Fixes #204

cdk-esbuild now defaults to esbuild@^0.15.0. This release of esbuild comes with Yarn PnP support, which is untested for this package (reports welcome!).

The package is still compatible with 0.14.* versions of esbuild. You might downgrade the version of esbuild using overrides:

// package.json
{
  "overrides": {
    "esbuild": "^0.14.0"
  }
}