Refactor: Standardize inconsistent keymap formats

[YousefHadder]

As part of the v2.0 update, we need to standardize the keymap definitions in lua/markdown-plus/table/keymaps.lua and lua/markdown-plus/headers/toc_window.lua to match the pattern used in keymap_helper.lua.

Current State:

• table/keymaps.lua manually defines <Plug> mappings and uses vim.keymap.set directly.
• headers/toc_window.lua has hardcoded buffer-local keymaps.

Desired State:

• All modules should utilize keymap_helper.setup_keymaps for consistency.
• Configuration should drive these keymaps.

Parent Issue: #180