Add a highlight for the keyword.storage scope to the onedark theme (#11802)

Rust highlight queries make use of keyword.storage for keywords like
`struct`, `enum`, and also for modifiers like `mut` and `ref`.

Using a color that's different to the one used for
`"variable.parameter"` (red) improves differentiation for mutable
function arguments.

runtime/themes/onedark.toml

@@ -15,6 +15,7 @@
 "keyword.control" = { fg = "purple" }
 "keyword.control.import" = { fg = "red" }
 "keyword.directive" = { fg = "purple" }
+"keyword.storage" = { fg = "purple" }
 "label" = { fg = "purple" }
 "namespace" = { fg = "blue" }
 "operator" = { fg = "purple" }