Adding trailing backslash rule

.vale/fixtures/OpenShiftAsciiDoc/TrailingBackslash/.vale.ini

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+; Vale configuration file to test the `TrailingBackslash` rule
+StylesPath = ../../../styles
+MinAlertLevel = suggestion
+[*.adoc]
+OpenShiftAsciiDoc.TrailingBackslash = YES

.vale/fixtures/OpenShiftAsciiDoc/TrailingBackslash/testinvalid.adoc

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+//vale-fixture
+[source,terminal]
+----
+oc get my lunch\
+----
+
+//vale-fixture
+[source,terminal]
+----
+C:\Program Files\
+----

.vale/fixtures/OpenShiftAsciiDoc/TrailingBackslash/testvalid.adoc

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+[source,terminal]
+----
+$ oc get my lunch
+----
+
+[source,terminal]
+----
+# oc get my lunch
+----
+
+.Example
+[source,terminal]
+----
+$ az role assignment create --role "<privileged_role>" \// <1>
+----
+
+[source,terminal]
+----
+sh-4.2# chroot /host
+----
+
+[source,terminal]
+----
+.
+├── CODEOWNERS
+├── README.md
+└── scripts
+    └── validate-vale-rules.sh
+----
+
+[source,terminal]
+----
+(undercloud)$ cd go/to/path
+----
+
+[source,yaml]
+----
+include::yaml/boundary-clock-ptp-config.yaml[]
+----
+
+.Example
+[source,terminal]
+----
+$ az role assignment \
+  create --role "<privileged_role>"
+----

.vale/styles/OpenShiftAsciiDoc/TrailingBackslash.yml

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+---
+extends: existence
+scope: raw
+level: error
+message: "Codeblock contains unescaped trailing backslash."
+action:
+  name: remove
+raw:
+  - '(?<!^----\n.+\n+\w+)(?<!\\)\\\n'