Import Base.Sys.isexecutable

On older versions of Julia, exporting our own `isexecutable()` was no
problem, because users rarely imported `Base.Sys`, however in Julia
v1.12+, `isexecutable` was moved from `Base.Sys` to `Base`, causing it
to be automatically imported by everyone, so we better merge our
methods.

Author: staticfloat

src/Abstract/ObjectHandle.jl

@@ -14,6 +14,7 @@ export MagicMismatch
 # Import methods for extension
 import Base: seek, seekstart, skip, position, read, readuntil, eof,
              getindex
+import Base.Sys: isexecutable
 import StructIO: unpack
 
 """