feat: add ignore file filter for referencing

[mhatzl]

List of issues that this PR closes

closes #19

Definition of Done ([req:qa.DoD])

Please consider the following requirements:

• Add/Update requirement references (see: [req:qa.tracing])

Note: You may ignore requirements that are not relevant to your PR.

Relevant decisions you made in this PR

Switched from crate walkdir to ignore, because it provides filtering using ignore files,
and provides a built-in iterator over all path entries.

.mantraignore is recognized, but takes precedence over others, because this is the behaviour of the ignore crate.
However, negating ignores made in .gitignore won't help if mantra commands are only run in workflows, because due to .gitignore, files and folders won't be pushed to the remote if ignored.

[github-actions]

---

mantra check ran successfully for branch: main

Checks:

- All references refer to existing requirements
- No deprecated requirement referenced
- No duplicate requirement IDs in wiki
- All entries in references lists are valid

1 new active requirement:

- req:filter references: 1

Took: 6ms