Allow Mithril to be loaded in non-browser environments without modification

[dead-claudia]

Description

Recast the global reads to all be guarded with typeof, so that if they aren't defined, they're just null.

Note that this does not involve documentation changes, as that's going to require a lot more effort and is best left as a follow-up. (Our support for this is undocumented anyways, and the relevant docs are for hooking it up with JSDOM. This only really matters in practice if you're using mithril-node-render.)

Motivation and Context

This has been a fairly common gotcha, and I'd like to actually fix it rather than repeatedly tell users how to work around it in about 2-3 different ways (depending on how they test their app). This in effect moves that workaround into core, but without polluting the global scope like #2613.

Closes #2613

How Has This Been Tested?

Ran require("./.") in the package root from a fresh Node REPL and no errors occurred.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation change

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.
• I have updated docs/changelog.md

[leontrolski]

I replaced this line
With "mithril": "git+https://github.com/isiahmeadows/mithril.js.git#bugfix", ran rm -rf node_modules; npm install; npm test and it worked like a charm. 👍

[orbitbot]

LGTM 👍 Can verify like @leontrolski that listed repro steps work as expected, but didn't do any wider verification as of right now wrt. actual functionality.

OT, re: PR branching, it might be a bit more straightforward to use individual names for each PR, but it's no biggie. Haven't really come to terms with git worktree across multiple upstream repositories yet, but that issue is mainly on this end 😉

[dead-claudia]

@orbitbot Regarding the branch name, I just lazied it. Normally, I'm more descriptive than this (and I prefer separate branches for each individual feature + fix), but yeah, I was lazy this time around and didn't feel like creating a whole new branch.