Add TypeScript type declarations (#23)

* feat: add ambient module stubs for declaration generation

Provide TypeScript ambient declarations for 'message_keys' (Pebble SDK
module) and './tmp/config-page.html' (build artefact) so tsc can resolve
these imports when generating .d.ts files.

* feat: add tsconfig for declaration generation

Configures tsc --allowJs --emitDeclarationOnly to generate per-file
.d.ts reference output in dist-types/ from the library source files.

* feat: add curated TypeScript declarations for public API

Hand-curated index.d.ts covering Clay, ClayConfig, ClayItem, ClayEvents,
manipulators, and all supporting types. Necessary because tsc cannot
infer the ClayEvents mixin or dynamically-bound manipulator methods.
All types are exported via the Clay namespace.

* test: add type-checking verification for index.d.ts

Exercises the full public API surface — constructor, instance/static
methods, ClayConfig/ClayItem event methods, manipulator methods,
namespace-exported types, and customFn this-context.

* build: add typescript devDependency, types field, and build scripts

Adds typescript 6.0.2 as a devDependency. Sets "types": "index.d.ts"
in package.json. Adds build:types and check:types scripts.

* build: add gulp types task to validate declarations on build

Validates index.d.ts against test/type-checks.ts as part of the default
gulp build. Reference declaration generation via build:types remains
available as a separate manual step.

* chore: update ignore files for TypeScript declarations

Add dist-types/ to .gitignore (build artefact). Whitelist index.d.ts
in .npmignore so type declarations ship with the package.

* ci: add typecheck workflow to validate TypeScript declarations

Runs npm run check:types on push and PR to main, matching the existing
lint/test/build workflow pattern.

package-lock.json

@@ -45,6 +45,7 @@
         "stringify": "^3.2.0",
         "through": "^2.3.8",
         "tosource": "^1.0.0",
+        "typescript": "6.0.2",
         "vinyl-buffer": "^1.0.0",
         "vinyl-source-stream": "^2.0.0",
         "watchify": "^3.11.1"
@@ -12014,6 +12015,20 @@
       "dev": true,
       "license": "MIT"
     },
+    "node_modules/typescript": {
+      "version": "6.0.2",
+      "resolved": "https://registry.npmjs.org/typescript/-/typescript-6.0.2.tgz",
+      "integrity": "sha512-bGdAIrZ0wiGDo5l8c++HWtbaNCWTS4UTv7RaTH/ThVIgjkveJt83m74bBHMJkuCbslY8ixgLBVZJIOiQlQTjfQ==",
+      "dev": true,
+      "license": "Apache-2.0",
+      "bin": {
+        "tsc": "bin/tsc",
+        "tsserver": "bin/tsserver"
+      },
+      "engines": {
+        "node": ">=14.17"
+      }
+    },
     "node_modules/ua-parser-js": {
       "version": "0.7.41",
       "resolved": "https://registry.npmjs.org/ua-parser-js/-/ua-parser-js-0.7.41.tgz",