Create .npmignore

### BEFORE
```sh
npm pack --dry-run

npm notice 📦  @11ty/[email protected]
npm notice === Tarball Contents ===
npm notice 147B  .editorconfig
npm notice 11B   .eleventyignore
npm notice 1.1kB LICENSE
npm notice 1.5kB .eleventy.js
npm notice 369B  src/absoluteUrl.js
npm notice 157B  sample/config-sample.js
npm notice 252B  src/dateRfc3339.js
npm notice 232B  test/dateToRfc3339Test.js
npm notice 230B  src/getNewestCollectionItemDate.js
npm notice 533B  test/getNewestCollectionItemDateTest.js
npm notice 603B  src/htmlToAbsoluteUrls.js
npm notice 1.4kB test/htmlToAbsoluteUrlsTest.js
npm notice 1.1kB package.json
npm notice 1.8kB README.md
npm notice 1.2kB sample/feed-json.njk
npm notice 1.3kB sample/feed.njk
npm notice 162B  sample/post.njk
npm notice 68B   .github/FUNDING.yml
npm notice === Tarball Details ===
npm notice name:          @11ty/eleventy-plugin-rss
npm notice version:       1.1.1
npm notice filename:      11ty-eleventy-plugin-rss-1.1.1.tgz
npm notice package size:  4.8 kB
npm notice unpacked size: 12.3 kB
npm notice shasum:        7c975f88d13e0a6e070908fba3faa21daa092b72
npm notice integrity:     sha512-pNjsK4CgypB7t[...]s1ejfaC/aTUSA==
npm notice total files:   18
```

### AFTER
```sh
npm pack --dry-run

npm notice 📦  @11ty/[email protected]
npm notice === Tarball Contents ===
npm notice 1.1kB LICENSE
npm notice 1.5kB .eleventy.js
npm notice 369B  src/absoluteUrl.js
npm notice 252B  src/dateRfc3339.js
npm notice 230B  src/getNewestCollectionItemDate.js
npm notice 603B  src/htmlToAbsoluteUrls.js
npm notice 1.1kB package.json
npm notice 1.8kB README.md
npm notice === Tarball Details ===
npm notice name:          @11ty/eleventy-plugin-rss
npm notice version:       1.1.1
npm notice filename:      11ty-eleventy-plugin-rss-1.1.1.tgz
npm notice package size:  3.1 kB
npm notice unpacked size: 7.0 kB
npm notice shasum:        024b61ab58c15a77b37ee81bf1875a7dab52ae4b
npm notice integrity:     sha512-XnVimTGWV6/9f[...]o78wrmRIJuclQ==
npm notice total files:   8
```

Author: pdehaan

.npmignore

@@ -0,0 +1,5 @@
+.editorconfig
+.eleventyignore
+.github/
+sample/
+test/