[ruby] Begin, Rescue, Ensure (Try-Catch Statements) for intermediate ast #3927

[badly-drawn-wizards]

The related issue assumes the RubyNode intermediate AST is complete for Begin, Rescue, Ensure, but this is not the case. The scope of this issue has crept to include handling the ANTLR-to-RubyNode AST feature for these expressions, as well as the AstCreator CPG building. This appears to be the case for a few of the other Ruby issues, which will influence their ETA accordingly.

Additionally, Rescue clauses in Ruby can contain splats which isn't handled yet in parsing. For now the exception list is will be an unknown block and posted as a medium-priority issue. I am unsure if adding a RubyNode for multipleRightHandSide affects anything else.

The scope of this PR is reduced to just the intermediate AST and the AST creator changes will be in a follow up.

[badly-drawn-wizards]

An open question for myself is that it that currently the same RubyNode can result in different AST generation depending on whether it is processed by astForExpression or astForStatement. For example

if true then end

results in a ControlFlowNode
and

x = if true then end

result in a ternary operation

This leads to the question of why this is the case and whether rescue blocks should follow the same logic or if all control flow can be treated uniformly as expressions.