Added has operator

pkg/yqlib/doc/Has.md

@@ -0,0 +1,44 @@
+This is operation that returns true if the key exists in a map (or index in an array), false otherwise.
+## Has map key
+Given a sample.yml file of:
+```yaml
+- a: yes
+- a: ~
+- a:
+- b: nope
+```
+then
+```bash
+yq eval '.[] | has("a")' sample.yml
+```
+will output
+```yaml
+true
+true
+true
+false
+```
+
+## Has array index
+Given a sample.yml file of:
+```yaml
+- []
+- [1]
+- [1, 2]
+- [1, null]
+- [1, 2, 3]
+
+```
+then
+```bash
+yq eval '.[] | has(1)' sample.yml
+```
+will output
+```yaml
+false
+false
+true
+true
+true
+```
+

pkg/yqlib/doc/headers/Has.md

@@ -0,0 +1 @@
+This is operation that returns true if the key exists in a map (or index in an array), false otherwise.