LSP, color scheme and picker updates

[johanvx]

Breaking changes

• Replace simrat39/rust-tools.nvim and nvim-telescope/telescope.nvim with rust-lang/rust.vim and echasnovski/mini.pick respectively.

  Note that echasnovski/mini.extra provides an abundance of pickers for mini.pick. Closes Replace telescope with mini.pick #40.

• Switch to nvimtools/none-ls.nvim, a community maintained version of jose-elias-alvarez/null-ls.nvim, as the original one is archived.

• Update the color scheme, including shrinking the color palette and tuning highlights.

  Note that the blue in the color palette is now actually blue, not indigo any more.

Features

• Use native inlay-hint provided by nightly Neovim.
• Use SmiteshP/nvim-navbuddy.
• Use relative line number by default.