Fix TypeScript case-sensitivity errors on case-insensitive filesystems

Fixes #637

Rename ALL type definition files to PascalCase for consistency with
their JS counterparts and module declarations:

Plugins:
- css-plugin.d.ts → CSSPlugin.d.ts
- css-rule-plugin.d.ts → CSSRulePlugin.d.ts
- custom-bounce.d.ts → CustomBounce.d.ts
- custom-ease.d.ts → CustomEase.d.ts
- custom-wiggle.d.ts → CustomWiggle.d.ts
- draggable.d.ts → Draggable.d.ts
- draw-svg-plugin.d.ts → DrawSVGPlugin.d.ts
- easel-plugin.d.ts → EaselPlugin.d.ts
- flip.d.ts → Flip.d.ts
- gs-dev-tools.d.ts → GSDevTools.d.ts
- inertia-plugin.d.ts → InertiaPlugin.d.ts
- morph-svg-plugin.d.ts → MorphSVGPlugin.d.ts
- motion-path-helper.d.ts → MotionPathHelper.d.ts
- motion-path-plugin.d.ts → MotionPathPlugin.d.ts
- observer.d.ts → Observer.d.ts
- physics-2d-plugin.d.ts → Physics2DPlugin.d.ts
- physics-props-plugin.d.ts → PhysicsPropsPlugin.d.ts
- pixi-plugin.d.ts → PixiPlugin.d.ts
- scramble-text-plugin.d.ts → ScrambleTextPlugin.d.ts
- scroll-smoother.d.ts → ScrollSmoother.d.ts
- scroll-to-plugin.d.ts → ScrollToPlugin.d.ts
- scroll-trigger.d.ts → ScrollTrigger.d.ts
- split-text.d.ts → SplitText.d.ts
- text-plugin.d.ts → TextPlugin.d.ts
- utils/velocity-tracker.d.ts → utils/VelocityTracker.d.ts

Update all references in types/index.d.ts accordingly.

The new exports field in package.json (3.14) with explicit type mappings
causes TypeScript to resolve types using the import path casing. On
case-insensitive filesystems, mismatches between reference paths and
import resolution trigger TS1149 errors.

Author: wietekepots-arch