Improve color scheme for Treesitter and scala

[merijn]

To customize the syntax highlighting of a capture, simply define or link a highlight group of the same name:

-- Highlight the @foo.bar capture group with the "Identifier" highlight group
vim.api.nvim_set_hl(0, "@foo.bar", { link = "Identifier" })

For a language-specific highlight, append the name of the language:

-- Highlight @foo.bar as "Identifier" only in Lua files
vim.api.nvim_set_hl(0, "@foo.bar.lua", { link = "Identifier" })

See :h treesitter-highlight-groups for details.