feat: support vtt in sync and fmt

docs/commands.md

@@ -107,6 +107,7 @@ sub-cli sync <source> <target>
 Currently, synchronization only works between files of the same format:
 - SRT to SRT
 - LRC to LRC
+- VTT to VTT
 
 ### Behavior Details
 
@@ -127,6 +128,16 @@ Currently, synchronization only works between files of the same format:
 - **Preserved from target**: All subtitle text content.
 - **Modified in target**: Timestamps are updated and entry numbers are standardized (sequential from 1).
 
+#### For VTT Files:
+
+- **When entry counts match**: Both start and end times from the source are directly applied to the target entries.
+- **When entry counts differ**: A scaled approach is used, similar to SRT synchronization:
+  - Start times are taken from proportionally matched source entries
+  - End times are calculated based on source entry durations
+  - The timing relationship between entries is preserved
+- **Preserved from target**: All subtitle text content, formatting, cue settings, and styling.
+- **Modified in target**: Timestamps are updated and cue identifiers are standardized (sequential from 1).
+
 ### Edge Cases
 
 - If the source file has no timing information, the target remains unchanged.
@@ -142,6 +153,9 @@ sub-cli sync reference.srt target.srt
 
 # Synchronize an LRC file using another LRC file as reference
 sub-cli sync reference.lrc target.lrc
+
+# Synchronize a VTT file using another VTT file as reference
+sub-cli sync reference.vtt target.vtt
 ```
 
 ## fmt

docs/zh-Hans/commands.md

@@ -107,6 +107,7 @@ sub-cli sync <源文件> <目标文件>
 目前，同步仅适用于相同格式的文件之间：
 - SRT到SRT
 - LRC到LRC
+- VTT到VTT
 
 ### 行为详情
 
@@ -127,6 +128,16 @@ sub-cli sync <源文件> <目标文件>
 - **从目标保留**: 所有字幕文本内容。
 - **在目标中修改**: 更新时间戳并标准化条目编号（从1开始顺序编号）。
 
+#### 对于VTT文件：
+
+- **当条目数匹配时**: 源的开始和结束时间直接应用于目标条目。
+- **当条目数不同时**: 使用缩放方法：
+  - 开始时间取自按比例匹配的源条目
+  - 结束时间根据源条目时长计算
+  - 保持条目之间的时间关系
+- **从目标保留**: 所有字幕文本内容和样式信息。
+- **在目标中修改**: 更新时间戳并标准化提示标识符。
+
 ### 边缘情况
 
 - 如果源文件没有时间信息，目标保持不变。
@@ -142,6 +153,9 @@ sub-cli sync reference.srt target.srt
 
 # 使用另一个LRC文件作为参考来同步LRC文件
 sub-cli sync reference.lrc target.lrc
+
+# 使用另一个VTT文件作为参考来同步VTT文件
+sub-cli sync reference.vtt target.vtt
 ```
 
 ## fmt