fix(tabufline): implement show_numbers option to display buffer index

[papier-krieger]

Problem

The show_numbers option is documented in nvchad_types/chadrc.lua and appears
in the tabufline config, but was never implemented. When set to true, no numbers
are displayed on the buffer tabs.

Additionally, the number shown was the internal Neovim bufnr (e.g. 6, 12, 24)
instead of the sequential buffer index (1, 2, 3...).

Solution

In style_buf(nr, i, w), the parameter i already receives the correct sequential
index from modules.lua, but was never used. This fix reads opts.show_numbers and
prepends the index i to the buffer tab label when enabled.

Changes

• lua/nvchad/tabufline/utils.lua: use parameter i to display buffer index when
  show_numbers = true