Add pkl:syntax module

[stackoverflow]

This new stdlib module allows for full manipulation of Pkl source code in Pkl: parsing text to a Pkl CST and formatting a Pkl CST back to text.

[odenix]

Have you considered offering the equivalent Java library? Wrapping that library instead of implementing pkl:syntax in Pkl would also improve (especially cold/warm) performance of pkl:syntax.

[HT154]

The major thing missing from this PR is the ability to whole-cloth construct and then format SyntaxNode instances without laboriously creating the underlying Nodes, especially given how common and important code generation is for the Pkl ecosystem. I'm not opposed to offering a Java API (pklpoet?) for this as long as the SyntaxNode classes remain instantiable (not external).

[stackoverflow]

The Java library is already Node. We still need the classes to be implemented in Pkl because we don't want to offer a generic API to users. As Jen mentioned, we don't want the classes to be external.