templater: add parsing and expansion rules to process "templatealias" section The debugtemplate command is updated to show expanded tree, but still the template engine doesn't support alias expansion. That's why the test says "parse error" for now.

templater: add function to parse whole string as template expression This will be a parser of template aliases, and it can also be used for processing quoted string templates in map files. That's why this function isn't defined in the upcoming _aliasrules class.