Add bower_components and dist to .npmignore.

[coreyfarrell]

Fixes #5244

[TimvdLippe]

Per #5244 (comment) using files is probably a better change. Could you use that instead?

Also: how can I test/verify this locally?

[coreyfarrell]

I've switched to using files, deleted .npmignore. This cleans a couple more files that are development only (detailed in commit message). Redundant entries are not listed, see npmjs docs for files that are automatically included. You can test this using npm pack, then inspect the tarball that is created.

With this change @polymer/polymer will be 1022kb unpacked. Of this CHANGELOG.md (automatically included by npm pack) is 324k, 31% of the total install. Maybe CHANGELOG.md should be split so it only includes the current semver-major, have a CHANGELOG-ARCHIVE.md or similar containing entries for legacy versions, add an a link to the end of the current changelog? Not included in this commit as decreasing from 14936kb to 1022kb is a huge improvement but I still feel it's worth considering not allowing the CHANGELOG.md included in the package to expand indefinitely.

Edit: note the sizes I've mentioned does not include any dependencies.

[TimvdLippe]

This is nice, thanks! We will keep the CHANGELOG for now, but we can revisit later.