Fix address ranges for i and a commands

[LoukasPap]

Fix issue 248

• Increase to 2 supported addresses for i (insert) and a (append) commands
• Add 1 simple test case for each

[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]

Same here. Prevent this under --posix and squash the commits. consider combining with the = command. Also update the README to document these extensions.

[LoukasPap]

@dspinellis You are right. I will add these in the PR #260.

Same here. Prevent this under --posix and squash the commits. consider combining with the = command. Also update the README to document these extensions.