feat: Replace mapFileDir argument with a function for reading the source map

[prantlf]

This is a part of changes to get this module usable in the web browser
by not depending on any Node.js modules.

• Remove the fs and path dependencies.
• Expect a function for reading the source map content to be passed as a parameter.
• Concatenate directory and file names without path.resolve.
• Behave synchronously or asynchronously depending if the source map reading method
  returned a string or a promise resolving to string.

BREAKING CHANGE: Methods fromMapFileComment and fromMapFileSource
require an additional parameter - readMap - to
read the source map content. They behave synchronously
or asynchronously depending on the behaviour
of the readMap function.

Fixes #64 with a breaking change on the API.

[phated]

@thlorenz I rebased this PR on all the changes in master and cleaned up this implementation so it no longer needs mapFileDir. Please take a couple minutes to review and let me know what you think. If this looks good, I think we can merge it and ship a 2.0.0

[phated]

This also closes #52 with my changes.

[thlorenz]

I very much like the end goal to make this work better in the browser.

However the way it is implemented currently will break a bunch of tools depending on this when running build scripts and such via Node.js.
The upgrade path is not trivial/obvious and thus I'm afraid most will just end up staying on the previous major if we publish as is.

I'd prefer a solution that keeps the old behavior working and warns with a deprecate message instead. We should also provide a utility function that basically is a read function that just takes a dir and thus makes getting rid of that deprecate message easier. The message obviously would point to the README where we explain how to use it.

Then in the next major version we can fully deprecate the old behavior.

[thlorenz]

Aside from that error printing nit LGTM.

Please improve that since without a stack users will be at a loss where this comes from especially when this module is a nested dep.

After that we can merge. Thanks for your work!

[phated]

Massive shoutout to @prantlf who did all the hard work on these PRs! Sorry it took us so long to get around to reviewing them but you've made this library a ton better 🙇