This commit creates a new compilation unit for the scripting engine code
by extracting the existing code from the functions unit.
We're doing this refactor to prepare the code for running the `EVAL`
command using different scripting engines.
This PR has a module API change: we changed the type of error messages
returned by the callback
`ValkeyModuleScriptingEngineCreateFunctionsLibraryFunc` to be a
`ValkeyModuleString` (aka `robj`);
This PR also fixes#1470.
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Signed-off-by: Ricardo Dias <ricardo.dias@percona.com>
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