Add build system; remove BigNumber.DEBUG; add toObject method

- Implement targeted builds (build.js) for ESM (.mjs), CommonJS (.cjs),
  and browser (.js) distributables in dist/, with matching type
  declarations (.d.mts, .d.cts, .d.ts).
- Remove BigNumber.DEBUG. The constructor now always throws on invalid
  input instead of returning NaN. isBigNumber always validates structure.
- Don't call toString on arbitrary objects passed to the constructor.
- Require value to be a string when a base is also passed.
- Add toObject prototype method.
- Add CI workflow and TypeScript type declaration import tests.
- Add .gitignore and .gitattributes; normalise line endings.
- Remove .npmignore; use 'files' field in package.json.
- Add typescript to devDependencies.
- Update README, API docs, and type declarations.

Author: MikeMcl

.gitattributes

@@ -0,0 +1,2 @@
+# Enforce LF normalization for all text files
+* text=auto

.github/workflows/ci.yml

@@ -12,8 +12,9 @@ jobs:
     runs-on: ubuntu-latest
 
     strategy:
+      # Build script requires Node.js >= 14.14.0.
       matrix:
-        node-version: ['8', '18', '20', '22']
+        node-version: ['14', '20', 'lts/*', 'node']
       fail-fast: false
 
     steps:
@@ -24,5 +25,31 @@ jobs:
         with:
           node-version: ${{ matrix.node-version }}
 
-      - name: Run Tests
-        run: node test/test
+      - name: Build
+        run: npm run build
+
+      - name: Run JS tests
+        run: npm test
+
+  typecheck:
+    name: Type-check
+    needs: test
+    runs-on: ubuntu-latest
+
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Use Node.js node
+        uses: actions/setup-node@v4
+        with:
+          node-version: 'node'
+
+      - name: Install dependencies
+        run: npm install
+
+      - name: Build
+        run: npm run build
+
+      - name: Type-check
+        run: npm run typecheck
+

.gitignore

@@ -0,0 +1,13 @@
+dist/
+node_modules/
+coverage/
+npm-debug.log*
+yarn-debug.log*
+yarn-error.log*
+package-lock.json
+npm-shrinkwrap.json
+.env
+.vscode/
+.idea/
+.DS_Store
+*.log