fix(md012): do not flag blank lines adjacent to headings (fixes #990).

Blank lines immediately above or below a heading are now excluded from
MD012's consecutive-blank-line count. Those blank lines are governed by
MD022 (blanks-around-headings), so MD012 defers to it. This means users
can set MD022's lines_above/lines_below > 1 without triggering spurious
MD012 violations.

test/no-multiple-blanks-headings.md

@@ -0,0 +1,23 @@
+# Heading
+
+
+## Two blank lines above
+
+Content.
+
+
+## Two blank lines above again
+
+More content.
+
+Text
+
+
+Text {MD012:15}
+
+Text
+
+
+## Three blank lines above
+
+Text

test/snapshots/markdownlint-test-scenarios.mjs.md

@@ -46267,6 +46267,54 @@ Generated by [AVA](https://avajs.dev).
       `,
     }
 
+## no-multiple-blanks-headings.md
+
+> Snapshot 1
+
+    {
+      errors: [
+        {
+          errorContext: null,
+          errorDetail: 'Expected: 1; Actual: 2',
+          errorRange: null,
+          fixInfo: {
+            deleteCount: -1,
+          },
+          lineNumber: 15,
+          ruleDescription: 'Multiple consecutive blank lines',
+          ruleInformation: 'https://github.com/DavidAnson/markdownlint/blob/v0.0.0/doc/md012.md',
+          ruleNames: [
+            'MD012',
+            'no-multiple-blanks',
+          ],
+          severity: 'error',
+        },
+      ],
+      fixed: `# Heading␊
+      ␊
+      ␊
+      ## Two blank lines above␊
+      ␊
+      Content.␊
+      ␊
+      ␊
+      ## Two blank lines above again␊
+      ␊
+      More content.␊
+      ␊
+      Text␊
+      ␊
+      Text {MD012:15}␊
+      ␊
+      Text␊
+      ␊
+      ␊
+      ## Three blank lines above␊
+      ␊
+      Text␊
+      `,
+    }
+
 ## no-multiple-blanks-maximum.md
 
 > Snapshot 1