Publish ESM + CJS + UMD modules

[TehShrike]

Fixes #58

Drops the hardcoded UMD pattern, allowing bundlers to avoid grabbing it.

Moves the isMergeableObject function out to its own module, allowing other libraries to use the same algorithm for iterating over objects (see #61).

Also fixed some test jank by removing unused variables and adding a missing var

[macdja38]

The is-mergable-object repository seems to be missing a license? It should probably be under:
https://github.com/KyleAMathews/deepmerge/blob/master/license.txt
Also it seems to calculate Object.prototype.toString.call(val) twice, this could possibly be made faster by storing the result in a variable?

[TehShrike]

I changed the is-mergeable-object license to MIT to match deepmerge, and broke those toString calls out to a function.

[macdja38]

My problem was not that the code was write twice... it was that it was run twice... breaking them out into a function call does not reduce the number of times they are run, it just ads the overhead of calling a function to them.

[TehShrike]

oh duh, lol. I'll fix that