ref: Bundler plugin core to just export helper utils (#116)

Refactor bundler plugin to a composable system for upcoming vite meta-framework support.

Author: nsdeschenes

.changeset/hungry-toes-hide.md

@@ -0,0 +1,8 @@
+---
+"@codecov/bundler-plugin-core": patch
+"@codecov/webpack-plugin": patch
+"@codecov/rollup-plugin": patch
+"@codecov/vite-plugin": patch
+---
+
+Refactor bundler-plugin-core to be a set of utility/helper functions, and refactor subsequent plugins to use these functions. This will make the plugins composable and easier to maintain in the future.