Extract the syntax tree from LIGO and use it for the debugger

In the debugger, we inherited the tree-sitter parsers from ligo-squirrel, but we'd like to eventually stop maintaining those and fetch the syntax tree from LIGO somehow.

Acceptance criteria

• We check which stage is the best to extract the syntax tree. I highly suspect the CST is the best one.
• LIGO has a way to serialize the syntax tree. JSON should do, but I worry the JSONs might become HUGE even for pretty small contracts. We should think of using a more efficient format, perhaps binary. We can, however, initially test JSONs to check for their sizes and make a prototype more easily.
• We remove the tree-sitter parsers and use the LIGO-provided syntax tree instead.
• All tests still pass.
• Some serialization tests are added to check we can correctly decode it.