Improve IDE quality of life using tsconfig paths

[jcready]

There are some minor annoyances when developing locally that this PR attempts to address:

1. In vscode (and likely other IDEs) when trying to "Go to definition" on something that was imported from another protobuf-ts package it will take you to the .d.ts file in the symlinked build folder. This makes navigating the repo more difficult than it need be.

   In an ideal world we would be able to use TypeScript's "Project References" which can handle automatically building other referenced packages when it detects a file has changed. Unfortunately ts-node (used for running tests, etc.) doesn't support this functionality: ts-node with project references in a lerna monorepo TypeStrong/ts-node#897

   So for now there is still a manual process of having to re-run make build in whatever package directory before re-running tests from another package directory which depended on that other package.

2. Improve karma code coverage results by turning on source maps during testing and ignoring the /spec directory (we don't need coverage results of the test suite itself).

3. Add top-level dev dependency for jasmine types so that you don't see red underlines on all the global jasmine functions: describe, test, it, expect, etc.

4. Add top-level dev dependency for typescript so that vscode (and likely other IDEs) can find/use the correct typescript lib (3.9.6) for inline type checking

[timostamm]

Agree with all changes, but have doubts around the tsconfig paths - let's give them a try though.