fix(colors): make whitespace chars visible

[amekusa]

Due to the color of whitespace characters (spaces and tabs) is same as the background color (#1F1F28), they are not visible unless being highlighted at the current line even if editor.renderWhitespace is not none.

So I changed editorWhitespace.foreground to be slightly lighter than editorIndentGuide.activeBackground #363646 but darker than
editorLineNumber.foreground #54546D.

This is the result: