fix(package): add exports map to package.json

[harshmandan]

Fixes #76

[metonym]

Thank you for this PR.

To add some context, I've backported a fix for v0.10.2 to include the exports map. This had to be done since the current master has breaking, unreleased changes.

However, this PR is still necessary to add an exports map.

[metonym]

This change can be undone since I manually add a separate commit for a version bump.

[metonym]

This is a good start, but more fields are necessary.

For example, an explicit entry point for types must be set. In addition, it's also nice to map subpaths (e.g., directly importing svelte-intersection-observer/src/IntersectionObserver.svelte). Without these, type errors will occur.

"exports": {
  ".": {
    "types": "./types/index.d.ts",
    "svelte": "./src/index.js"
  },
  "./src/*.svelte": {
    "types": "./src/*.svelte.d.ts",
    "import": "./src/*.svelte"
  },
  "./src/*": {
    "types": "./src/*.d.ts",
    "import": "./src/*.js"
  }
},