Allow any keys to drive navigation

[spenthil]

Motivation

Ctrl+h/j/k/l costs real defaults in some setups: Neovim's Ctrl+l clear-and-redraw, and readline's Ctrl+l / Ctrl+k in shell panes (the README already calls this out). Some people also navigate with other chords entirely — I use ctrl+arrow, matching my tmux bindings on servers. This PR makes every key configurable while changing no defaults.

Changes

• navigate.sh: the key forwarded into a Vim pane now reads HERDR_NAV_KEY_LEFT/DOWN/UP/RIGHT per direction, defaulting to the previously hardcoded ctrl+h/j/k/l.
• editor/nvim.lua + editor/vim.vim: navigation is exposed as <Plug>(HerdrNavigateLeft/Down/Up/Right). g:herdr_nav_keys replaces any subset of the default keys, and an empty string drops that direction. g:herdr_nav_no_mappings = 1 defines no keys at all, so any binding can drive the <Plug> mappings directly.
• README.md: a "Custom keys" section documents the three hops that must agree (herdr keybind → forwarded key → editor map), with a complete ctrl+arrow example. One over-long intro sentence is split; no other prose changes.

Compatibility

With nothing set, behavior is identical: same keys, same forwarding. The one observable difference is that :map shows the default keys as <Plug> redirections instead of direct function calls.

Testing

• navigate.sh against a live herdr 0.7.5 pane running Neovim: a probe mapping in that Neovim recorded the forwarded chord — ctrl+h with nothing set, and ctrl+up with HERDR_NAV_KEY_UP='ctrl+up'.
• Both editor files in a headless nvim --clean matrix: defaults, custom keys, a dropped direction, and no_mappings, checking the resulting maps with maparg(). A feedkeys() test confirms a custom chord moves between two real splits through the <Plug> chain.

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