feat(view): support padding around border characters

[hat0uma]

Summary

This adds support for configuring virtual spacing on both sides of column delimiters.

Users can now write:

require("csvview").setup({
  view = {
    spacing = {
      left = 1,
      right = 1,
    },
  },
})

This makes crowded column-border layouts easier to read.

view.spacing = 2 (Default and existing behavior)	view.spacing = { left = 1, right = 1 } (New spacing mode)

Changes

• Extend view.spacing to accept either:
  • a number, preserving the existing behavior
  • a table, { left = ..., right = ... }, for delimiter-aware spacing
• Split field rendering padding into:
  • alignment padding, used to keep numeric/text columns aligned
  • delimiter spacing, used to add visual room around column borders
• Add coverage for:
  • highlight display mode
  • border display mode
  • multiline fields
• Update README and GUIDE with the new table-style spacing option

Notes

This does not add a new column-alignment configuration option yet. It preserves the current behavior where numeric fields are right-aligned and other fields are left-aligned, while making the spacing around delimiters configurable.

#94