feat: add JetBrains IDE support via TextMate bundle and LSP4IJ 🧩

[timfennis]

Why

.ndc files had no editor support in JetBrains IDEs (RustRover). Prompted by wanting syntax highlighting in RustRover while reusing the existing ndc lsp server instead of writing a native IntelliJ plugin.

What

• Syntax highlighting — no new grammar: JetBrains' bundled TextMate Bundles plugin imports the existing VS Code extension directory (ext/andy-cpp) directly. The grammar gains three small additions while here: complex literals (3i, 2.5j), #! shebang comments, and ? in function definition names (fn square?).
• ext/lsp4ij-ndc/template.json — LSP4IJ config that launches ndc lsp --stdio for *.ndc files, giving diagnostics, hover, completion, inlay hints, and go-to-definition from the existing LSP server.
• Docs — setup instructions added to the manual's editor-support page; README links there from a new "JetBrains IDEs" section.

Notes for reviewers

• An earlier revision added a standalone grammar under ext/textmate-ndc; it was removed in favour of reusing ext/andy-cpp so there is a single grammar to maintain.
• The grammar changes are additive; the published VS Code extension will pick them up on its next release.

🤖 PR description created by Claude