feat: add snapshot reports

[camshaft]

Description of changes:

This change adds snapshot report functionality.

Before this change, codebases were left with the binary option of enforcing full coverage of requirements or not with the --ci flag. This isn't ideal, since it assumes that all of the requirements are covered already. As such, we need something to preserve the expected state of the requirement coverage in time and then assert it matches what we want it to in CI.

Codebases wanting to use this functionality would first enable the snapshot reports in their config:

[report.snapshot]
enable = true

Next, they run duvet report. This will write a snapshot file to .duvet/snapshot.txt. They should check this in to version control to ensure it stays in sync with the codebase.

Finally in CI, running duvet report --ci will ensure the current state of the requirement coverage matches the snapshot file. If not, an error is returned and the job fails:

duvet/integration/snapshots/report-snapshot-out-of-date_stderr.snap

Lines 5 to 33 in 5e0b2b9

	$ duvet report --ci
	EXIT: Some(1)
	Extracting requirements
	Extracted requirements from 1 specifications
	Scanning sources
	Scanned 1 sources
	Parsing annotations
	Parsed 1 annotations
	Loading specifications
	Loaded 1 specifications
	Mapping sections
	Mapped 1 sections
	Matching references
	Matched 1 references
	Sorting references
	Sorted 1 references
	Writing .duvet/reports/report.html
	Wrote .duvet/reports/report.html
	Writing .duvet/snapshot.txt
	Differences detected in .duvet/snapshot.txt:
	@@ -1 +1,3 @@
	SPECIFICATION: [Section](my-spec.md)
	+ SECTION: [Section](#section)
	+ TEXT[implementation]: here is a spec
	× .duvet/snapshot.txt
	╰─▶ Report snapshot does not match with CI mode enabled.

The snapshot format is a very simple text format that works well with tools like git diff. It consists of the following structure:

SPECIFICATION: [Spec Title](path/to/spec)
  SECTION: [Section Name](#section-name)
    TEXT[implementation,test]: Text goes here
    TEXT[implementation]: Other text goes here

  SECTION: [Other Section Name](#other-section-name)
    TEXT[!SHOULD,todo]: Not implemented

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