Buildkite plugin: exports use inconsistent casing

[Mike Ball]

It'd be nice (and less confusing) if the buildKiteApiRef were exported as buildkiteApiRef.

Feature Suggestion

To establish more consistent and expected character casing with the plugin's other
exports, the BK API ref could be exported as buildkiteApiRef. The current
ref export, buildKiteApiRef, is a bit odd and unexpected.

Consistent character casing would improve the adoption experience for other users,
particularly those who want to use the plugin's BK API in their own
plugin development.

Possible Implementation

To preserve backwards compatibility, the plugin could also continue to export buildKiteApiRef.