Add --properties-separator option (#1951)

This commit adds the --properties-separator option, which lets users
specify the separator used between keys and values in the properties
output format. This is done by adjusting the value of
github.com/magiconair/properties#Properties.WriteSeparator at encode
time.

Some refactoring of the properties encoder unit tests was done to make
it easier to write unit tests that include different separator values.

Fixes: #1864

Signed-off-by: Ryan Drew <ryan.drew@isovalent.com>

pkg/yqlib/doc/usage/properties.md

@@ -120,6 +120,36 @@ emptyArray =
 emptyMap = 
 ```
 
+## Encode properties: use custom separator
+Provide a custom key-value separator using the `--properties-separator` flag.
+
+Given a sample.yml file of:
+```yaml
+# block comments come through
+person: # neither do comments on maps
+    name: Mike Wazowski # comments on values appear
+    pets: 
+    - cat # comments on array values appear
+    food: [pizza] # comments on arrays do not
+emptyArray: []
+emptyMap: []
+
+```
+then
+```bash
+yq -o props --properties-separator=";" sample.yml
+```
+will output
+```properties
+# block comments come through
+# comments on values appear
+person.name;Mike Wazowski
+
+# comments on array values appear
+person.pets.0;cat
+person.food.0;pizza
+```
+
 ## Decode properties
 Given a sample.properties file of:
 ```properties