Check why core/ is included in blockly npm package

[cpcallen]

Describe the bug

While debugging test failures in google/blockly-samples#1452 I noticed that the blockly npm package v9.2.0 contains all of core/**/*.ts, which is unexpected as I thought build_tasks.js had been modified so that the only source code included in the package was in the form of sourcemaps.

I should investigate to see if this is a regression, an error on my part in the previous changes, a deliberate reversion of the previous change (and if so for what reason), or if these files continue to be included for some good reason I was aware of at the time but no longer recall.

[cpcallen]

It turns out that (at least as of now, in develop) the only files in the core/ directory of the package (i.e., in dist/core/ after running npm run package) are .d.ts files. So we are not shipping our TypeScript source code (except for the copy embedded in the .js.map files, as intended).