Replace duplicate files with a build script

[hurrymaplelad]

Neat project! I'd like to contribute more. These changes should make that easier.

See README.md for new build and test instructions.

Committing copies of files like password_template.html complicates merging and rebase, which bit me while experimenting with the webcrypto branch.

I added a package.json files allowlist to keep the new build script from bloating the published package.

I verified the files that would be published with npm run build && npm package --dry-run:

=== Tarball Contents === 
npm notice 1.1kB  LICENSE               
npm notice 10.2kB README.md             
npm notice 47.9kB crypto-js.min.js      
npm notice 9.0kB  index.js              
npm notice 1.2kB  package.json          
npm notice 9.8kB  password_template.html

[hurrymaplelad]

This was redundant. .staticrypt.json matches subdirectories as well.

[robinmoisson]

Hey @hurrymaplelad - thanks for your PRs! That's all looking great. I've been bitten bitten by the duplicated files and been wanting to automate all of that for a while, and really like your approach here.