Support border color, width and style in default and custom highlights

[3noix]

This PR resolves #582 by adding support of:

• border color
• border width
• border style for default and custom highlights.

I treated the border color similarly to foreground and background colors:

• possibility to use a color scheme
• possibility to use theme colors values

Default values:

• border color: the background color
• border width: 0
• border style: solid

Here is the result of a test:

With these settings in todo-tree.highlights.customHighlight and no foreground and background set in todo-tree.highlights.defaultHighlight:

[3noix]

I had a problem with the VSCode API: I only wanted to add a horizontal line above my comment, without applying a background or a foreground change, and I had a foreground in the todo-tree.highlights.defaultHighlight... and in this case it is impossible to reference the color of comments of the theme:

• editor.tokenColorCustomizations.comments:
  • shows a warning when settings.json is saved (because invalid)
  • same result as if "editor.foreground" was used
• editor.foreground:
  • no warning
  • not the color of comments
• hard coded hex value: works obviouslybut will not change with the theme
• null:
  • shows a warning when settings.json is saved (because invalid)
  • has the expected effect

And this is some weird behavior that 2 different invalid values (as said by the warning) have different results.