fix(package): includes types in exports map

[metonym]

Fixes #311

[metonym]

After some more testing, I've found by trial and error that this works best.

The explicit *.js extension is needed for Rollup. Same for *.css.

".": {
  "types": "./index.d.ts",
  "svelte": "./index.js"
},
"./*": {
  "types": "./*.d.ts",
  "svelte": "./*.svelte"
},
"./package.json": "./package.json",
"./styles/*.css": {
  "import": "./styles/*.css"
},
"./styles/*": {
  "types": "./styles/*.d.ts",
  "import": "./styles/*.js"
},
"./languages/*": {
  "types": "./languages/*.d.ts",
  "import": "./languages/*.js"
}