Ark: Argument completions should not evaluate special bindings

For instance here: https://github.com/posit-dev/amalthea/blob/73d03d16/crates/ark/src/lsp/completions/session.rs#L481. We take steps to avoid evaluating complex expressions but within functions simple symbols may trigger arbitrary computations via lazy evaluation.

Currently low priority but will become more important when completions within the debugger are supported (#1871). This causes a timeout panic if the freeze lasts for more than 5 seconds, so this should definitely be fixed at some point.

Reprex:

# Simulate an argument that takes a long time when forced
delayedAssign("foo", Sys.sleep(10))

# Requesting argument completions freezes R and the LSP
foo(<CURSOR>)