chore: add files field to package.json

[thescientist13]

Realized in our demo repo that we were shipping the entire contents of the repo to NPM

This change now only ships the src/ directory

➜  eleventy-plugin-wcc git:(master) ✗ npm pack
npm notice
npm notice 📦  @projectevergreen/eleventy-plugin-wcc@0.3.0
npm notice Tarball Contents
npm notice 2.7kB README.md
npm notice 954B package.json
npm notice 1.2kB src/index.js
npm notice 1.5kB src/utils.js
npm notice Tarball Details
npm notice name: @projectevergreen/eleventy-plugin-wcc
npm notice version: 0.3.0
npm notice filename: projectevergreen-eleventy-plugin-wcc-0.3.0.tgz
npm notice package size: 2.7 kB
npm notice unpacked size: 6.3 kB
npm notice shasum: 57169991c6decaec1af30e37a37ea5ea16b71fb7
npm notice integrity: sha512-kPVrq0okymM5f[...]j3ZsNoISHwicQ==
npm notice total files: 4
npm notice
projectevergreen-eleventy-plugin-wcc-0.3.0.tgz
➜  eleventy-plugin-wcc git:(master) ✗ tar -xvcf projectevergreen-eleventy-plugin-wcc-0.3.0.tgz
➜  eleventy-plugin-wcc git:(master) ✗ ls -la package
total 16
drwxr-xr-x   5 owenbuckley  staff   160 Dec 31 20:47 .
drwxr-xr-x  24 owenbuckley  staff   768 Dec 31 20:47 ..
-rw-r--r--   1 owenbuckley  staff   954 Oct 26  1985 package.json
-rw-r--r--   1 owenbuckley  staff  2659 Oct 26  1985 README.md
drwxr-xr-x   4 owenbuckley  staff   128 Dec 31 20:47 src
➜  eleventy-plugin-wcc git:(master) ✗ ls -la src 
total 16
drwxr-xr-x   4 owenbuckley  staff   128 Dec 31 20:32 .
drwxr-xr-x  24 owenbuckley  staff   768 Dec 31 20:47 ..
-rw-r--r--   1 owenbuckley  staff  1219 Dec 29 21:16 index.js
-rw-r--r--   1 owenbuckley  staff  1515 Dec 31 20:32 utils.js

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