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.json

@@ -2,12 +2,15 @@
   "name": "@rebble/clay",
   "version": "1.0.8",
   "description": "Pebble Config Framework",
+  "types": "index.d.ts",
   "scripts": {
     "test-travis": "./node_modules/.bin/gulp && ./node_modules/.bin/karma start ./test/karma.conf.js --single-run --browsers chromeTravisCI",
     "test-debug": "(export DEBUG=true && ./node_modules/.bin/gulp && ./node_modules/.bin/karma start ./test/karma.conf.js --no-single-run)",
     "test": "./node_modules/.bin/gulp && ./node_modules/.bin/karma start ./test/karma.conf.js --single-run",
     "lint": "./node_modules/.bin/eslint ./",
     "build": "gulp",
+    "build:types": "tsc -p tsconfig.declarations.json",
+    "check:types": "tsc -p tsconfig.typecheck.json",
     "dev": "gulp dev",
     "pebble-clean": "rm -rf tmp src/js/index.js && pebble clean",
     "pebble-publish": "npm run pebble-clean && npm run build && pebble build && pebble package publish && npm run pebble-clean",
@@ -85,6 +88,7 @@
     "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"