Permit to use custom CSS

[RobinDev]

This feature permit to add Custom CSS directly in user or workspace settings.json by adding a configuration property.

"markdown-editor.customCss": "my custom css rules"

// Eg: "markdown-editor.customCss": ".vditor-ir pre.vditor-reset {line-height: 32px;padding-right: calc(100% - 800px) !important; margin-left: 100px;    font-family: system-ui !important;}"

Fix #83 Fix #28 Fix #46

Maybe more issues...