Add block params as first class node in V2 syntax tree

There are at least three cases where it would be helpful to have a first class node for block params:

1. Codemods, where renaming a block parameter may need to be done
2. Linting, where the name of a block parameter/variable may be linted against (e.g. if it conflicts with keywords)
3. Throwing syntax errors, as in the work done in glimmerjs/glimmer-vm#1206

We should add them to the v2 syntax tree in a way that allows us to easily access their `loc`, so errors can be informative and helpful for users.