Add TypeScript build config for .d.ts declaration output

Set up tsconfig.json (declaration: true, DOM lib), package.json with
main/types/files pointing at dist, and .gitignore for node_modules/dist
so the library can be consumed with type declarations.

tsconfig.json

@@ -0,0 +1,12 @@
+{
+  "compilerOptions": {
+    "target": "ES2020",
+    "module": "ESNext",
+    "moduleResolution": "Bundler",
+    "lib": ["ES2020", "DOM"],
+    "declaration": true,
+    "outDir": "dist",
+    "strict": true
+  },
+  "include": ["abstract-table.ts"]
+}