Add separate Node and Bun entrypoints

[mherod]

Summary

Add explicit Node and Bun entrypoints so the package can expose runtime-appropriate APIs instead of shipping a single Node-oriented build.

Motivation

The package already documents Bun installation in README.md:44-45, but it currently publishes only one ESM entrypoint via main and types in package.json:6-7. The build also emits only src/index.ts today (package.json:13, tsup.config.ts:4-9). Runtime-facing modules import Node APIs directly (src/mdfind.ts:1-3, src/mdls.ts:1-2, plus src/mdutil.ts, src/mdimport.ts, src/live-search.ts, src/query-builder.ts, and src/discover.ts), so there is no dedicated seam for Bun-specific process execution.

Proposed Behavior

• Expose distinct package entrypoints for Node and Bun, backed by a shared public API surface.
• Introduce an interop layer that abstracts process execution and stream handling so Node implementations can use node:child_process while Bun entrypoints can use Bun-native APIs where appropriate.
• Update package exports and build output so consumers can import the correct runtime entrypoint explicitly and TypeScript resolves the matching declarations.
• Document the supported import paths and runtime expectations.

Acceptance Criteria

• package.json uses an exports map for runtime-specific entrypoints instead of only main/types.
• tsup builds separate outputs for the shared surface plus Node/Bun-specific entry files.
• Command execution is routed through a shared interop boundary rather than importing node:child_process directly in each public module.
• Public docs and examples show how to import and use the Node and Bun entrypoints.
• Test coverage verifies both entrypoints expose the intended API surface and execute Spotlight commands correctly on macOS.

Implementation Notes

• package.json:6-7 and package.json:13 currently publish/build a single dist/index.js output.
• tsup.config.ts:4-9 defines only one build entry (src/index.ts).
• src/index.ts:1-36 is a single barrel export today and likely needs sibling runtime entry barrels.
• src/mdfind.ts:1-3 and src/mdls.ts:1-2 show the current direct Node API dependency pattern that the interop layer should absorb.
• No similar open issues were found from searches for bun, entrypoint exports, or node bun.