fix: set underline for indent-blankline ContextStart

[daephx]

I think that the highlight group IndentBlanklineCurrentContext should either be removed in order to use the plugins default settings, or should be changed to use an underline that matches the current context color set by #50.

This PR currently does the latter and preserves IndentBlanklineCurrentContext but sets underline = true and switches guifg with guisp for vscContextCurrent.

With indent_blankline.setup({ show_current_context_start = true }), vscode.nvim sets a foreground color for IndentBlanlineContextStart, resulting in treesitter highlights being overridden with colors.vscContextCurrent.

Using tokyonight as a comparison: IndentBlanklineCurrentContext isn't set at all, implying that the default settings from indent-blankline.nvim are used.

[daephx]

Previous screenshots were in Windows Terminal which doesn't handle underline colors very well from what I can tell.

Here are some examples within kitty terminal.

With highlight vscContextStart:

With IndentBlanklineContextStart removed, highlight gets set to: IndentBlanklineContextStart xxx cterm=underline gui=underline guisp=#c586c0