Release to fix bazel build layering issue in 3.21.3.

This release consists of mostly minor changes:

• Documentation link fixes
• Updated code generator to work with upstream API changes
• Groundwork for moving to Bazel modules

This is a hotfix release for #141

• Enable Closure's advanced optimization mode by default. This should reduce code size (#124).
• Replace goog.testing.asserts in tests with Jasmine expectations (#136).
• Bring in the subset of the goog.asserts we use as jspb.asserts. This code retains an Apache 2.0 license.
• Fix decoding of signed varint32s (#31).
• Add license files to NPM package.

This will also be our first release to have cross-compiled protoc-gen-js binaries. Please note that they use dynamic linking (similar to the protobuf protoc releases).

First release of protobuf-javascript as a standalone project.

This release mostly consists of BUILD and documentation fixes to account for new repository.
Various bug fixes.