Add an experimental SQLITE_ENABLE_UNKNOWN_SQL_FUNCTION compile-time option.

Add an experimental SQLITE_ENABLE_UNKNOWN_SQL_FUNCTION compile-time option.
When enabled, the "unknown function" error is suppressed for EXPLAIN and
a no-op function named "unknown" is substituted.  This facilitiates using
the command-line shell to analyze queries from applications that contain
many application-defined functions that are not normally available to the
shell.

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