feat: Add BracketTransform mode (replace Telex 2)

[hthienloc]

Summary

• Remove Telex 2 input method definition
• Add BracketTransform mode (FollowFlags, Disabled, NonStart, Everywhere)
• Independent of input method (works with Telex, VNI, VIQR, etc.)
• Add SetBracketTransformMode() to IEngine interface
• Correctly handle upper/lower case transformations ([ -> ơ/Ơ, ] -> ư/Ư)

Changes

• bamboo.go: Implement BracketTransform logic and update IEngine interface.
• input_method_def.go: Remove Telex 2 and clean up [] / {} mappings.

[gemini-code-assist]

Code Review

This pull request introduces a configurable bracket transformation feature to the Bamboo engine, allowing square brackets to be automatically converted to Vietnamese characters (ơ, ư) based on new engine modes and flags. Additionally, it refines and renames several input method definitions, such as merging Telex, VNI, and VIQR layouts. Review feedback identifies a logic error in the SetBracketTransformMode range check that prevents resetting to follow flags, and highlights that changing the VIQR mapping for 'Đ' from 'd' to '\' may break common typing shortcuts like 'dd'.