feat: make keymaps configurable via setup()

[Cannon07]

Summary

• Adds a keys table to setup() so the previously hardcoded mappings (]c, [c, <leader>dq) can be customized or disabled
• Always defines <Plug>(CodePreviewCloseAll) for users who prefer to bind the close action themselves
• Defaults match the existing hardcoded values, so behavior is unchanged for current users

Closes #43

Usage

require("code-preview").setup({
  keys = {
    next_change = "]c",          -- inline diff: next change
    prev_change = "[c",          -- inline diff: previous change
    close_all   = "<leader>dq",  -- close diff and clear indicators
  },
})

Set any entry to false to skip that binding, or keys = false to skip them all and use <Plug>(CodePreviewCloseAll) directly.

Test plan

• Defaults (]c/[c/<leader>dq) work unchanged across Claude Code, OpenCode, and GitHub Copilot CLI backends
• Custom keys (e.g. <leader>cn/<leader>cp/<leader>cq) bind correctly and the original defaults no longer trigger
• keys = false disables all defaults; <Plug>(CodePreviewCloseAll) still bindable manually
• Per-key false (e.g. close_all = false) skips just that binding