Editor title border (#177)

VS Code 1.45 [1] (April 2020) introduced new theme keys for render a
border below the editor group header [2] (for example, below breadcrumbs
if enabled) to restore the previous behavior of
`editorGroupHeader.tabsBorder`.

This commits adapts the new theme key to match the style of
`editorGroupHeader.tabsBorder`.

[1]: https://code.visualstudio.com/updates/v1_45
[2]: https://code.visualstudio.com/updates/v1_45#_new-theme-color-for-editor-title-borde

Resolves GH-176

Author: arcticicestudio

themes/nord-color-theme.json

@@ -61,6 +61,7 @@
     "editorGroup.background": "#2e3440",
     "editorGroup.border": "#3b425201",
     "editorGroup.dropBackground": "#3b425299",
+    "editorGroupHeader.border": "#3b425200",
     "editorGroupHeader.noTabsBackground": "#2e3440",
     "editorGroupHeader.tabsBackground": "#2e3440",
     "editorGroupHeader.tabsBorder": "#3b425200",