Improve TypeError messages for non-numeric/empty strings in coercions; Closes #20632

[rizwan3d]

Implements proposal from #20632

[iluuu1994]

Please also see my comment in #20632 (comment).

[iluuu1994]

Not all non-empty strings are non-numeric. You can see this in the tests, it says non-numeric for many examples where one of the strings is actually numeric.

[rizwan3d]

To address the issue of numeric strings being reported as "non-numeric string", would adding an is_numeric_string_ex() check like this in zend_zval_numeric_string_value_name() be sufficient?

	zend_long lval;
        double dval;

		bool trailing_data = false;
        if (is_numeric_string_ex(Z_STRVAL_P(val), Z_STRLEN_P(val),&lval, &dval, 0, NULL, &trailing_data)) 
		{
            return "string";
        }

Ref.: ext/opcache/jit/zend_jit_helpers.c:1189
case IS_STRING:
		{
			bool trailing_data = false;
			/* For BC reasons we allow errors so that we can warn on leading numeric string */
			if (IS_LONG == is_numeric_string_ex(Z_STRVAL_P(dim), Z_STRLEN_P(dim), &offset, NULL,
					/* allow errors */ true, NULL, &trailing_data)) {
				if (UNEXPECTED(trailing_data)
				 && EG(current_execute_data)->opline->opcode != ZEND_FETCH_DIM_UNSET) {
					zend_error(E_WARNING, "Illegal string offset \"%s\"", Z_STRVAL_P(dim));
				}
				return offset;
			}
			zend_illegal_container_offset(ZSTR_KNOWN(ZEND_STR_STRING), dim, BP_VAR_R);
			return 0;
		}

The idea is that empty strings still map to "empty string", non-empty non-numeric strings to "non-numeric string", and numeric strings fall back to "string" so they’re not mislabelled.