plscope_identifiers: ensure name_usage does not overflow
plscope_identifiers: ensure name_usage does not overflow

This limits the length of the name_usage column to at most 250
characters, wrapping the value to the left if necessary. Without
this there remained a small possibility to hit the ORA-01489
"result of string concatenation is too long" exception when
computing that value, at least in principle.

PhilippSalvisberg/plscope-utils,PhilippSalvisberg/plscope-utils