Compartment mapper should take first matching of conditional exports

[kriskowal]

Describe the bug

Node.js package conditional imports should take the entries under the first matching tag and ignore all subsequent tags. This is hinted with the express advice that default must appear last if present.

Steps to reproduce

Create a bundle from a package that has conditional exports e.g., endo and default and note that default wins.

Expected behavior

The first match (node in the example) should win.

[kriskowal]

This fix is necessary for progress toward native XS support #400.