refactor: external tools and shells

* rename Models.ExternalMergeTools to Models.ExternalMerger
* supports Git Bash/PowerShell/Command Prompt/Default Shell in Windows Terminal

src/Resources/Locales/en_US.axaml

@@ -266,9 +266,9 @@
   <x:String x:Key="Text.Preference.Git.Email" xml:space="preserve">User Email</x:String>
   <x:String x:Key="Text.Preference.Git.Email.Placeholder" xml:space="preserve">Global git user email</x:String>
   <x:String x:Key="Text.Preference.Git.Path" xml:space="preserve">Install Path</x:String>
+  <x:String x:Key="Text.Preference.Git.Shell" xml:space="preserve">Shell</x:String>
   <x:String x:Key="Text.Preference.Git.User" xml:space="preserve">User Name</x:String>
   <x:String x:Key="Text.Preference.Git.User.Placeholder" xml:space="preserve">Global git user name</x:String>
-  <x:String x:Key="Text.Preference.Git.UsePowershellOnWindows" xml:space="preserve">Use Powershell instead of Git bash</x:String>
   <x:String x:Key="Text.Preference.Git.Version" xml:space="preserve">Git version</x:String>
   <x:String x:Key="Text.Preference.GPG" xml:space="preserve">GPG SIGNING</x:String>
   <x:String x:Key="Text.Preference.GPG.Enabled" xml:space="preserve">Commit GPG signing</x:String>

src/Resources/Locales/zh_CN.axaml

@@ -266,9 +266,9 @@
   <x:String x:Key="Text.Preference.Git.Email" xml:space="preserve">邮箱</x:String>
   <x:String x:Key="Text.Preference.Git.Email.Placeholder" xml:space="preserve">默认GIT用户邮箱</x:String>
   <x:String x:Key="Text.Preference.Git.Path" xml:space="preserve">安装路径</x:String>
+  <x:String x:Key="Text.Preference.Git.Shell" xml:space="preserve">终端Shell</x:String>
   <x:String x:Key="Text.Preference.Git.User" xml:space="preserve">用户名</x:String>
   <x:String x:Key="Text.Preference.Git.User.Placeholder" xml:space="preserve">默认GIT用户名</x:String>
-  <x:String x:Key="Text.Preference.Git.UsePowershellOnWindows" xml:space="preserve">使用PowerShell替代Git Bash</x:String>
   <x:String x:Key="Text.Preference.Git.Version" xml:space="preserve">Git 版本</x:String>
   <x:String x:Key="Text.Preference.GPG" xml:space="preserve">GPG签名</x:String>
   <x:String x:Key="Text.Preference.GPG.Enabled" xml:space="preserve">启用提交签名</x:String>