Add exports property to package.json

[Stuk]

Fixes #104

NodeJS in ESM mode, and Typescript under Node16 resolution do not read the module property of package.json. This leads to errors using micro-memoize with Node16 resolution in Typescript as it doesn't think the types match the CommonJS implementation (even though this does work in most bundlers and Node today). You can see the error on this debug page.

This PR adds the exports field that points to the mjs version of the library, with adjacent index.d.ts satisfying both Node and TS.

You may be able to view this modification running here, and see Typescript's resolution process by running npm run build:

======== Resolving module 'micro-memoize' from '/home/projects/vitejs-vite-tyb3tz/src/main.ts'. ========
Explicitly specified module resolution kind: 'Node16'.
Resolving in ESM mode with conditions 'import', 'types', 'node'.
File '/home/projects/vitejs-vite-tyb3tz/src/package.json' does not exist according to earlier cached lookups.
File '/home/projects/vitejs-vite-tyb3tz/package.json' exists according to earlier cached lookups.
Loading module 'micro-memoize' from 'node_modules' folder, target file types: TypeScript, JavaScript, Declaration.
Searching all ancestor node_modules directories for preferred extensions: TypeScript, Declaration.
Directory '/home/projects/vitejs-vite-tyb3tz/src/node_modules' does not exist, skipping all lookups in it.
Found 'package.json' at '/home/projects/vitejs-vite-tyb3tz/node_modules/micro-memoize/package.json'.
Using 'exports' subpath '.' with target './mjs/index.mjs'.
File name '/home/projects/vitejs-vite-tyb3tz/node_modules/micro-memoize/mjs/index.mjs' has a '.mjs' extension - stripping it.
File '/home/projects/vitejs-vite-tyb3tz/node_modules/micro-memoize/mjs/index.mts' does not exist.
File '/home/projects/vitejs-vite-tyb3tz/node_modules/micro-memoize/mjs/index.d.mts' exists - use it as a name resolution result.
'package.json' does not have a 'peerDependencies' field.
Resolving real path for '/home/projects/vitejs-vite-tyb3tz/node_modules/micro-memoize/mjs/index.d.mts', result '/home/projects/vitejs-vite-tyb3tz/node_modules/micro-memoize/mjs/index.d.mts'.
======== Module name 'micro-memoize' was successfully resolved to '/home/projects/vitejs-vite-tyb3tz/node_modules/micro-memoize/mjs/index.d.mts' with Package ID 'micro-memoize/mjs/[email protected]'. ========