feat: add motions and textobjects

[hat0uma]

This PR introduces new motion- and textobject-related APIs designed to enhance CSV navigation and selection in csvview.nvim.
The two primary additions are:

1. A flexible field() function in jump.lua, enabling cursor movements based on CSV row/column coordinates (both relative and absolute).
2. A convenient field() textobject in textobject.lua, allowing you to select the current CSV field (with optional delimiter inclusion).

Additionally, several supporting changes are included:

• New jump-based APIs in jump.lua (e.g., next_field_end, prev_field_end, etc.) that behave like Vim's native motions (w, b, e, ge) but specifically for CSV fields.
• A new get_cursor() function in util.lua that retrieves the cursor’s position with CSV-awareness (i.e., which row/column in the CSV, whether on a delimiter, etc.).

---

1. field() in jump.lua

-- Basic usage:
require("csvview.jump").field(0, {
  pos = { 1, 2 },      -- Move to row=1, column=2
  mode = "absolute",   -- "relative" or "absolute"
  anchor = "start",    -- "start" or "end" of the field
  col_wrap = true,     -- Wrap horizontally to next/previous row if needed
})

• Relative Movements (default): pos = { row_offset, col_offset } from the current field.
• Absolute Movements: mode = "absolute" interprets pos as 1-based row/column.
• Anchor: If "start", place the cursor at field start; "end" jumps to field end.
• Column Wrapping: Wrap to the previous/next row when exceeding column bounds.

For convenience, there are also shorthand motions such as:

next_field_start(bufnr?) -- like `w` motion.
prev_field_start(bufnr?) -- like `b` motion.
next_field_end(bufnr?) -- like `e` motion.
prev_field_end(bufnr?) -- like `ge` motion.

---

2. field() textobject in textobject.lua

-- Basic usage in an operator-pending or visual context:
require("csvview.textobject").field(0, {
  include_delimiter = false
})

• Select Current Field: Selects the entire CSV field under the cursor.
• Delimiter Inclusion: Set include_delimiter = true if you want to include the trailing or preceding comma in your selection.

References: #16