Comprehensive build & test instructions

[eoghanmurray]

Given the new monorepo structure, the following is my understanding:

• you have to go into each subpackage in order to run the build commands (yarn bundle). This definitely wouldn't be obvious on first download of project
• building the rrweb package depends on building rrweb-snapshot package, which is only a problem whenever there are changes to the latter. The relationship between these and how rrweb includes rrweb-snapshot isn't clear, see don't reference old rrweb-snapshot  #725
• yarn typings is great, but it is not included in the regular test scripts. New devs might miss this as a requirement
• there are multiple different target outputs that take time to build/test but may not be relevant for an individual developer

So I'm proposing that we have a set of top level scripts in the project root which accomplishes the above, and generates usable javascript output files for each.

The ones I'm interested in are:
./build-for-web.sh - compiles rrweb-snapshot and rrweb but ignores es and other outputs
(above could be split into separate record/replay scripts if that saved compilation time
./test.sh - runs tests over any packages that have changes. recompiles rrweb-snapshot if it has changed and runs rrweb tests given the dependency between them

Problem is I'm not sure how best to do it!