Add Clear and ClearAll methods to WebGLInput

[dannykoh]

Summary
Implemented two new methods for input field management.

• Clear() — Resets the text content of a single input field on both Unity and HTML sides.
• ClearAll() — Clears all input fields simultaneously.

Details
These methods provide a programmatic way to reset input fields, ensuring that users start with a clean state when entering new data. Without invoking these methods, previously entered text values may persist across sessions and unintentionally override new inputs.

[kou-yeung]

Thanks for the PR! 🙏

WebGLInput is designed so that just attaching the component should handle syncing automatically —
usually setting InputField.text = "" or TMP_InputField.text = "" clears the HTML side when the user starts typing again.

Could you clarify why adding Clear() / ClearAll() was needed?

• Does .text = "" fail to sync in some cases?
• If so, could you share the conditions or steps to reproduce it (e.g. focus lost, UI re-open, DOM not yet created)?
• Any specific Unity or browser environment involved?

Understanding that would help decide whether we need a new API or just improve the existing sync behavior.