Add off-by-default rule for unused call results

[nitishagar]

Introduce an unused-call-result diagnostic that fires when a bare
expression statement is a function/method call whose result is discarded
and the result type is informative (not None, Any, or Never).

The check sits in binding_to_type_stmt_expr as an else if branch
after the existing unused-coroutine check, so the two diagnostics
are mutually exclusive: a discarded coroutine call still emits only
unused-coroutine. The guard matches!(e, Expr::Call(_)) ensures
assignments, bare names, comparisons, and similar non-call statements
are not flagged. Results of type Any or that extend Any are also
exempted, consistent with the coroutine rule. The rule defaults to
Severity::Ignore and must be explicitly enabled.

A new test file covers every spec boundary: the golden-path discard,
assigned/None/Any/Never/extends-Any exemptions, method and constructor
calls, non-call bare statements, the R5 coroutine mutual-exclusion case
(rule ON + coroutine discard → exactly one unused-coroutine error),
and the default-off regression.

Fixes #3719

Test Plan:

• cargo test -p pyrefly_config test_doc_headers test_doc_severities
• cargo test -p pyrefly --lib unused_call_result
• cargo test -p pyrefly --lib simple::test_unused_coroutine
• python3 test.py --no-test --no-conformance --no-jsonschema

[github-actions]

According to mypy_primer, this change doesn't affect type check results on a corpus of open source code. ✅