flexible line terminators

### Is your feature request related to a problem? Please describe

Hi,

Could asdf open up the line terminator sequences supported in .tool-version files?

* LF (modern)
* CRLF (Windows)
* degenerate mixtures of LF/CRLF in poorly formatted text files

In asdf 0.15 and earlier, we seem to get subtle problems with .tool-version files that contain CRLF.

Perhaps a newer version has already addresed this. But I though I'd ask on behalf of users stuck on some older release versions.

### Describe the proposed solution

Use a generic buffered line reader to perform line termination delimiting in an OS agnostic way.

To mockup an AST regex, `/\r*\n+/m`

### Describe similar `asdf` features and why they are not sufficient

Not applicable.

### Describe other workarounds you've considered

asdf is peerless!

Start by locating the .tool-version parsing entry point and reproduce the reported behavior with LF, CRLF, and mixed line terminators. Done means asdf reads all three forms consistently, including files from older release versions, without changing other version-file behavior.

Written by the indexing model from the issue text.