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:

[Lamarcke]

I honestly believe this is intentional. Most editors don't have editor whitespace colors, and some people don't like them.

[amekusa]

Hm, If it's intentional, that's ok then. Maybe I will create a fork for my own.
But in my opinion, if you don't like whitespaces are visible, that is editor.renderWhitespace setting for.