When a document is opened or changed, scan for `type Name { field: Type }` blocks
and `let var: Type` / `fn foo(param: Type)` annotations. On completion requests,
if the text before the cursor ends with `identifier.`, look up the variable's type
and return its fields as Field (kind=5) completion items instead of the full list.
Implements a Language Server Protocol server for El files over stdin/stdout
JSON-RPC. Provides completions (builtins + keywords + document functions),
hover documentation, go-to-definition, and full document sync.
Also adds a VSCode extension that launches the binary as a child process and
a TextMate grammar for .el syntax highlighting.
NOTE: el-lsp.el calls __read_n(n: Int) -> String, a seed primitive not yet
in el_runtime.c. Build.sh documents the required C implementation; the seed
agent must add it before the binary will link.
Will Anderson