Support braced-init-lists with `mkNode()` (#6580)

Support braced-init-lists with `mkNode()` (#6580)

This commit adds support for braced-init-lists in calls to `mkNode()`,
e.g., `mkNode(REGEXP_EMPTY, {})`. Previously, such a call would result
in a node of kind `REGEXP_EMPTY` with a single null node as a child
because the compiler chose the `mkNode(Kind kind, TNode child1)` variant
and converted `{}` to a node using the default constructor. This commit
adds an overload of `mkNode()` that takes an `initializer_list<TNode>`
to allow this use case. It also adds a `mkNode()` overload with zero children
for convenience and removes the 4- and 5-children variants because they
saw little use. Finally, it makes the default constructor of `NodeTemplate`
explicit to avoid accidental conversions.