feat(config): make listsym_rejected configurable (P2)

The rejected/cancelled checkbox glyph was a hardcoded `const REJECTED = '-'`.
ListSyms now carries a `rejected` field (new_with_rejected); a per-wiki
`listsym_rejected` key (default `-`) is added to WikiConfig, and
WikiConfig::list_syms() builds the palette with it. Routed the three
config-driven ListSyms construction sites (parse path in lib.rs + the
toggle/cycle commands) through list_syms(), so a custom rejected glyph both
lexes and round-trips.

Tests: listsyms::custom_rejected_glyph_is_honoured + config round-trip.
README/doc/lua config comment updated. fmt clean; 0 Rust test failures.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

crates/nuwiki-lsp/src/lib.rs

@@ -193,7 +193,7 @@ impl Backend {
     fn parse_for_uri(&self, uri: &Url, text: &str) -> nuwiki_core::ast::DocumentNode {
         let syms = self
             .wiki_for_uri(uri)
-            .map(|w| nuwiki_core::listsyms::ListSyms::new(&w.config.listsyms))
+            .map(|w| w.config.list_syms())
             .unwrap_or_default();
         nuwiki_core::syntax::vimwiki::VimwikiSyntax::new().parse_with_listsyms(text, &syms)
     }