Add Nextflow language server support (#1815)

The Nextflow language server schedules a debounced (1s) AST update on every
didOpen/didChange, and LanguageService.references is the only request that does
not await it -- documentSymbol, codeLens, documentLink and semanticTokensFull
all do. A references request issued inside that window races the recompile of
the file it asks about and can return an empty list, which is what the macOS CI
runner hit. Send a documentSymbol request for the same file first, which blocks
server-side until the pending update has been applied.

Also compare reference paths using the platform separator in the tests, since
relativePath is OS-native and the hardcoded forward slashes failed on Windows.

.github/workflows/pytest.yml

@@ -66,7 +66,7 @@ jobs:
     # test can fall through to "no batch". To add/rebalance: edit a MARKERS_* var, and (for a new
     # group) add a case branch below + the batch name to the matrix list.
     env:
-      MARKERS_JVM: "java or csharp or fsharp or kotlin or scala or groovy or clojure or bsl"
+      MARKERS_JVM: "java or csharp or fsharp or kotlin or scala or groovy or clojure or bsl or nextflow"
       MARKERS_NATIVE: "cpp or rust or go or zig or pascal or swift"  # swift is macOS-only (skips elsewhere via the guard)
       # other-langs: grab-bag + scripting languages + haskell + qml. Haskell (GHC/HLS) and qml (qmlls) are
       # installed only on Linux -- too slow/expensive to build on the macOS runner (Haskell), or not worth