更新variant文档，支持默认变体

Author: pirunxi

docs/manual/variants.md

@@ -93,6 +93,8 @@ return {
 
 在为某个变量指定了当前使用的变体后，如果对应的字段不存在，则取默认字段的值。同样地，以变体en为例，如果`value@en`列不存在，则取`value`列的值，如果`value`列也不存在，则抛出字段找不到的错误。
 
+可以使用`--variant default={variantName}`设置全局默认变体，当未没有为某个bean单独设置变体时，则取全局默认变体。
+
 如果某个字段定义了变体，但没有在命令行中使用`--variant`指定该字段使用的变体名，则读取不含变量的原始变量的值。此时Luban会打印一行警告日志。
 
 

i18n/en/docusaurus-plugin-content-docs/current/manual/variants.md

@@ -92,4 +92,6 @@ Generally speaking, since a variable variant is defined, the variant that should
 
 After specifying the currently used variant for a variable, if the corresponding field does not exist, the value of the default field is taken. Similarly, taking variant en as an example, if the `value@en` column does not exist, the value of the `value` column is taken. If the `value` column does not exist either, an error that the field cannot be found is thrown.
 
+You can set a global default variant using --variant default={variantName}, and when a variant is not individually set for a bean, the global default variant will be used.
+
 If a field has a variant defined, but the variant name used by the field is not specified using `--variant` in the command line, the value of the original variable without the variant is read. At this time, Luban will print a warning log.