Add combine operator to template language

[RhetTbull]

See this reddit post. The ability to combine two multi-value templates into a new list of values would be useful inside the template language.

New syntax:
`"pretext{delim+template_field:subfield(field_arg)|filter[find,replace] conditional&combine_value?bool_value,default}posttext"

&combine_value: Template fields may be combined with another template statement to return multiple values. The combine_value is another template statement. For example, the template {created.year&{folder_album,}} would resolve to ["1999", "Vacation"] if the photo was created in 1999 and was in the album Vacation. Because the combine_value is a template statement, multiple templates may be combined together by nesting the combine operator: {template1&{template2&{template3,},},}. In this example, a null default value is used to prevent the default value from being combined if any of the nested templates does not resolve to a value

Another use case for variables is filtering combined template values. For example, using the &combine_value mechanism to combine two template values that might result in duplicate values, you could do the following: {var:myvar,{template1&{template2,},}}{%myvar|uniq} which allows the use of the uniq filter against the combined template values.