Add initial implementation of BOM management classes and data structure

- Created clsBOMManager for managing BOM data
- Added clsCategory and clsMaterialItem for category and material representation
- Implemented methods for loading and organizing BOM data from Excel
- Introduced .gitignore to exclude unnecessary files
- Added metadata JSON for module tracking

VBA/ClassModules/clsMaterialItem.cls

@@ -0,0 +1,12 @@
+' ========================================
+' 类模块: clsMaterialItem
+' 用途: 表示单个物料项
+' ========================================
+Option Explicit
+
+Public code As String           ' 代号
+Public Name As String           ' 名称
+Public Quantity As Double       ' 数量
+Public Condition As String      ' 选择条件
+Public Category As String       ' 类别
+Public ParentCategory As String ' 上层类别