Add fhir-shorthand extension

[spilikin]

Adds FHIR Shorthand (fhir-shorthand), a language extension for
FHIR Shorthand (FSH) — the HL7 DSL used to
author FHIR Implementation Guides (compiled by
SUSHI).

It provides syntax highlighting and a code outline for .fsh files via a
purpose-built Tree-sitter grammar ported from SUSHI's official ANTLR grammar
(FSH 3.0.0).

Repositories

• Extension: https://github.com/spilikin/zed-fhir-shorthand
• Grammar: https://github.com/spilikin/tree-sitter-fsh

Tested

Installed locally via zed: install dev extension against the published
grammar at the pinned rev; highlighting and outline (cmd-shift-o) work
across all FSH 3.0.0 constructs (Profile:, Extension: with Context:,
Logical: with Characteristics:, Instance:, Invariant:, ValueSet:,
CodeSystem:, Mapping:, RuleSet:/insert, Alias:, CodeableReference()).

Checklist

• Extension and grammar repositories are public on GitHub
• LICENSE at the extension repo root (MIT)
• Extension ID does not contain zed / Zed / extension
• Grammar pinned by a full commit SHA
• Submodule uses an HTTPS URL
• Submodule commit is on a branch (main), not detached
• version matches between extension.toml and extensions.toml
• pnpm sort-extensions and pnpm test pass locally

🤖 Generated with Claude Code

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[spilikin]

@cla-bot check

[cla-bot]

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[spilikin]

Hi all, any chance you accept this pull request? We would like to start using it in the FHIR community.

Thanx in advance!