Fix truncated pattern space problem

[LoukasPap]

Fix issue 253

• Initialize the context of the hold space to \n (newline character)
• Previously, it started as an empty string which resulted to wrong outputs

The solution fixes the problem for the swap pattern x but also for g and G.

[github-actions]

GNU sed testsuite comparison:

Test results comparison:
  Current:   TOTAL: 64 / PASSED: 8 / FAILED: 56 / SKIPPED: 0
  Reference: TOTAL: 64 / PASSED: 8 / FAILED: 56 / SKIPPED: 0

[dspinellis]

Please fix the failing tests, ensuring the output is compatible with existing sed output. Also ensure and test that the behavior is correct under --separate.

Probably best to wait for the integration of this PR and then add the code in the following block.

        if index == 0 || context.separate {
            context.line_number = 0;
            reset_latched_address_ranges(&mut context.range_commands);
        }

[LoukasPap]

Yes, I'll get back on this when the PR is accepted.