Modify build process to disable code splitting

The code splitting feature in `tsup` breaks up source code into logical
modules so that if a module is not being used it can be removed by a
build tool that supports tree shaking.

However, the modules that `tsup` creates do not necessary match the
files in the original source code, and `tsup` gives them a random name.
This makes it frustrating for developers on a client team who are
debugging a `core` package by inspecting the compiled version, because
it is impossible to tell what a chunk file contains without first
opening it up.

This commit thus configures `tsup` to disable the code splitting
feature.

tsup.config.ts

@@ -38,7 +38,7 @@ const config: Options = {
 
   // Split the output into chunks. This is useful for tree-shaking.
   // https://tsup.egoist.dev/#code-splitting
-  splitting: true,
+  splitting: false,
 };
 
 export default config;