Duration Literals should support ISO 8601 notation

__Proposal:__
Duration Literals should support ISO 8601 notation. It should facilitate the development of libraries, integrations and query parametrization.

__Current behavior:__
Duration Literals have a custom syntax

__Desired behavior:__
Describe what you want.

__Alternatives considered:__
- Implement a custom function that parse ISO8601 into Duration
- Develop libraries in every language to parse/generate the current Duration Literal syntax

__Use case:__
Will be easier to develop libraries that generate/parametrize queries.

The issue names no files, tests, or entry points. First clarify the intended ISO 8601 duration grammar and query-language compatibility, then locate the duration-literal parser and its tests; done should include agreed syntax, parsing behavior, and regression coverage.

Written by the indexing model from the issue text.