Issue #551 : Accept callable tables as the first argument of xpcall

[XinyuGuan]

Fixes #551.

baseXPCall now uses L.CheckAny(1) instead of L.CheckFunction(1), allowing callable tables to reach the existing protected-call machinery. pcall already supports callable tables following #328.

Extra arguments remain ignored, preserving Lua 5.1 behavior. A supplied non-callable first argument now returns false plus the error handler's result instead of raising an argument-check error, matching Lua 5.1.5's control flow. Diagnostic text may differ.

The regression test in _glua-tests/issues.lua covers callable-table success and error handling.

Validation:

• make build
• go test -count=1 ./
• git diff --check

Related: #467 also removes this check as part of a broader argument-forwarding change. This PR isolates the callable-value fix while keeping the current behavior of not forwarding extra arguments.