Fix exports field in package.json

[fisker]

Fixes #350

A solution that postss used seems unnecessary, as I tested, use "./*": "./*" works on Node.js 12 too.

[fisker]

@kpdecker Can you take a look?

[ExplodingCabbage]

A solution that postss used seems unnecessary, as I tested, use "./": "./" works on Node.js 12 too.

I think you probably tested on 12.20 or later, but that it's necessary for earlier versions? The history at https://nodejs.org/docs/latest-v12.x/api/packages.html#packages_modules_packages says that support for patterns (i.e. use of *) in exports was only added in version 12.20. Some testing confirms this; if I install jsdiff from local source with npm install --save ../jsdiff and try require("diff/lib/patch/create.js"), in Node 12.19 or earlier I need ./ in the exports section or else I get:

$ node
Welcome to Node.js v12.19.1.
Type ".help" for more information.
> require("diff/lib/patch/create.js")
Uncaught:
Error [ERR_PACKAGE_PATH_NOT_EXPORTED]: Package subpath './lib/patch/create.js' is not defined by "exports" in /home/mark/marktest2/node_modules/diff/package.json
    at throwExportsNotFound (internal/modules/esm/resolve.js:285:9)
    at packageExportsResolve (internal/modules/esm/resolve.js:491:3)
    at resolveExports (internal/modules/cjs/loader.js:444:36)
    at Function.Module._findPath (internal/modules/cjs/loader.js:484:31)
    at Function.Module._resolveFilename (internal/modules/cjs/loader.js:818:27)
    at Function.Module._load (internal/modules/cjs/loader.js:687:27)
    at Module.require (internal/modules/cjs/loader.js:903:19)
    at require (internal/modules/cjs/helpers.js:74:18)
    at repl:1:1
    at Script.runInThisContext (vm.js:120:18) {
  code: 'ERR_PACKAGE_PATH_NOT_EXPORTED'
}

So let's go for the postcss solution of including both ./ and ./*, just for maximum compatibility.