Expose meta information about built in type converters as public api for compiler plugin

We don't have extensibility here except for Parsers (String -> Something). So List -> Int is always an error. Would be nice to be able to generate a map of supported converters and use it for diagnostic there

`dataFrameOf("a" to columnOf(listOf(1, 2, 3))).convert { a }.toInt()`

We'd be able to filter some obvious errors, like selected File column being converted toDouble, etc. Seems minor, but interesting. Ideally it should be something lightweight, Map?

Review the parser-based converter behavior and compiler plugin context described in the issue; no source files or tests are named. Define the supported converter metadata and public API, then verify diagnostics for invalid conversions such as List→Int and File→Double.

Written by the indexing model from the issue text.