feat: add volar.virtualFiles.mappingSelectionBackgroundColor setting

[Sight-wcg]

close #2147

default

• dark
  
• light
  

configured by string

"volar.virtualFiles.mappingSelectionBackgroundColor":  "#F8C9AB"

• light
  

configured by theme

// https://code.visualstudio.com/api/references/theme-color#editor-colors
"volar.virtualFiles.mappingSelectionBackgroundColor": {
  "id": "editor.selectionBackground"
},

• dark
  

• light
  

[johnsoncodehk]

LGTM, thanks!

The setting is not needed because you already add the light background color.