fix: renumber Unicode rule from E020 to E012 for sequential numbering

[jgamblin]

This PR fixes the rule numbering to be sequential without gaps.

Changes

• Changed Unicode escape sequence rule from E020 to E012
• Eliminates the gap between E011 and the next rule
• Allows dependent PRs (Add PURL validation support (Issue #17) #20 and Add CNA Rules v4.0 validation checks (Issue #13) #21) to use sequential numbering (E013-E014 for PURL, E015-E020 for CNA)

Why This Matters

PR #19 (Unicode validation) was merged with E020, creating a gap. This should have been E012 to keep numbering sequential. With this fix:

• E001-E011: Original rules
• E012: Unicode
• E013-E014: PURL (PR Add PURL validation support (Issue #17) #20)
• E015-E020: CNA Rules v4.0 (PR Add CNA Rules v4.0 validation checks (Issue #13) #21)

This fix is needed before merging PRs #20 and #21.

[jgamblin]

Not needed - PRs #20 and #21 have been updated to work with the current upstream/main state where Unicode remains as E020. The renumbering to E012 is no longer necessary.