refactor: taskfile/ast package

[pd93]

I've always found it a little strange that the code that reads a Taskfile is in a read package. I'm used to domain-driven packages, so it feels more correct to me to have something like taskfile.Read(...) than read.Taskfile(...). This PR is a proposal to rearrange the API a little bit to something I think is a bit nicer.

Instead of the schema being taskfile.Taskfile, we have a taskfile/ast (Abstract Syntax Tree) package which hosts all the code for parsing and storing the Taskfile syntax. The taskfile package is then responsible for interfacing with Taskfiles (Read(...), Exists(...), Merge(...) etc).

Note: I based this branch on remove-v2-support (#1447), so that needs to be merged first.

[andreynering]

Nice refactor! 👏

ast could also have been called schema, but both options are more than fine!

[pd93]

ast could also have been called schema, but both options are more than fine!

I did consider schema, but I felt that ast was more accurate as the structures don't always directly correspond to the schema. Rather, they are an abstraction of it. It's also less characters for my lazy fingers to type 🤣

I don't feel strongly about this though if we want to change it before the next release!