Replace current line based parsing

[mholthausen]

Maybe consider a replacement for the current line based parser, as mentioned in this comment

[geographika]

@mholthausen - a few notes you might find useful.

There is a full Mapfile grammar in Python at https://github.com/geographika/mappyfile/blob/master/mappyfile/mapfile.lalr.g which uses Lark. It has been tested on all the Mapfiles that are used in the MapServer CI testing.

Lark has a converter from nearly to Lark, but not currently the other way round. However

These might get added in the future, if enough users ask for them.

https://github.com/lark-parser/lark/blob/master/docs/tools.md

So it might be worth considering https://github.com/kach/nearley as the parsing engine so the same source grammar could be used.