feature: Add highlight for IndentBlanklineContextStart

[dotfrag]

Did you check the docs?

• I have read all the tokyonight.nvim docs

Is your feature request related to a problem? Please describe.

Currently there is no theme-native highlight for IndentBlanklineContextStart. This would be useful for the following configuration:

return {
  "lukas-reineke/indent-blankline.nvim",
  opts = {
    show_current_context = true,
    show_current_context_start = true,
  },
}

I'm using LazyVim but this should apply to everyone. Currently, it shows an underline (default):

Describe the solution you'd like

I'd like to have a IndentBlanklineContextStart highlight built-in to the theme that you believe matches the aesthetic the best.

Describe alternatives you've considered

Currently I'm using a dirty workaround (I know it doesn't follow the plugin's override option)

-- indent-blankline current context highlight
cmd([[highlight IndentBlanklineContextStart guibg=#363c58]])

Additional context

This was inspired by NvChad's default config which highlights the current context using indent-blankline.

Thank you for considering!