node: Introduce new module for working with postgres Node types

[urso]

Requires #67

Closes #66

Postgres sources use tagging and the Node (and NodeTag) types for many internal types. Unfortunately the macros did not translate well to Zig, which makes it annoying to work with the low level types.

We introduce the pgzx.node module that provides a list of helper functions like pgzx.node.make, pgzx.node.create, pgzx.node.tag, or pgzx.node.isA.

We also wrap the constants into our own pgzx.node.Tag enum, so you can use pgzx.node.isA(node, .Query) in code.

Postgres uses code generation to collect the node tags and supported types into nodetags.h. We also use code generation to produce the list of tags and type mappings. The generated module is then imported by pgzx.node.

The tools/gennodetags tool imports the nodes/nodes.h header file at compile time and uses comptime to collect all known tags into an ArrayList which is then used to produce the sources for the anonymous module imported into pgzx.