signatures: reject BreakpointRef and stray nothing program counters

[aviatesk]

step_expr! and next_until! return a BreakpointRef when a breakpoint is hit or when JuliaInterpreter.break_on(:error)/break_on(:throw) is active and a statement throws. The method-definition walkers in signatures.jl never handled that case (nor, on some paths, nothing), so pc was inferred as Union{Int,Nothing,BreakpointRef} and, when it did happen, failed later with an unrelated error. Concretely, with break_on(:error) enabled (e.g. via Debugger.jl) Revise reported ArgumentError: invalid index: breakpoint(...) instead of the actual UndefVarError for a typo in a method signature, because Revise's disablebp only disables breakpoint instances, not break_on.

Add throw_if_breakpoint, which rethrows the error carried by the BreakpointRef (what would have propagated without break_on) or raises an error otherwise, and narrows pc to Union{Int,Nothing}. Wrap every step_expr!/next_until! call site with it, handle the remaining nothing cases explicitly (step_through_methoddef, get_running_name, the inner-method loop of methoddef!), and let _methoddefs! accept nothing.

Along the way: assert the MethodInfoKey returned by signature for a 3-arg :method, drop the redundant trailing frame.pc = pc (both step_expr! and next_or_nothing! already advance it, and it would throw for nothing), and report the "no method found" warning at the :method statement pc3 rather than the statement after it.

JET(LS) no longer reports any pc-related issue in signatures.jl; the LoweredCodeUtils and Revise test suites pass.