Migrates most of the files under src/core from Closure type-checking to Typescript. Closure will no longer pass type-checking on many (most?) of these files. As a result, all Closure-based type-checking will stop passing (and so has been silenced in check-types.js). Later PRs will handle cleaning out legacy type-checking code.

Two #core directories are excluded: context and assert. context is intricate and isolated, so it can be migrated independently while this PR unblocks progress elsewhere in the repo. assert will be more involved because of differences in how CC and TS handle assertion annotations, so its type signatures are stubbed in this PR to be addressed in a later PR.

Some "globals" we access through the window object, which we rely on for some polyfills and testing scenarios. Because TS doesn't expect them to be on the window, there are corresponding TS definition files .d.ts which add these properties to the Window interface. Anything in a .d.ts file is TS-only logic that need to live outside of existing JS annotations. Mostly, they replace existing .extern.js files.

Files ending in .shame.d.ts contain "band-aids", such as type stubs, that allow type-checking to pass. One example is assert.shame.d.ts, which tells tsc to skip type-checking #core/assert for now and instead provides the type signatures it should expect.