Rename parse_node -> node_id (#3760)

This was previously discussed at
https://discord.com/channels/655572317891461132/655578254970716160/1209975051588210729.
I'm initiating this mainly because we typically use "id" suffixes to
indicate an `IdBase` being passed around and the non-id suffix of
`parse_node` suggests at it carrying more data than it actually does.
There used to be more reason for avoiding `node_id` because
`SemIR::InstId` used to be named `NodeId`, but that's no longer
necessary. As a consequence, I'd like to rename `parse_node` to more
precisely reflect its type.

In full, this is doing:

```
parse_node_kind -> node_kind
parse_node -> node_id
ParseNodeCategory -> NodeCategory
ParseNodeKind -> NodeKind
ParseNode -> NodeId
```

This is primarily in check and sem_ir, but with some `parse_node_kind`
references in parse too.

Pluralization is consistent with name forms on both sides, so that
wasn't part of my replacements.

toolchain/check/handle_expr_statement.cpp

@@ -15,14 +15,14 @@ static auto HandleDiscardedExpr(Context& context, SemIR::InstId expr_id)
   // If we discard an initializing expression, convert it to a value or
   // reference so that it has something to initialize.
   auto expr = context.insts().Get(expr_id);
-  Convert(context, context.insts().GetParseNode(expr_id), expr_id,
+  Convert(context, context.insts().GetNodeId(expr_id), expr_id,
           {.kind = ConversionTarget::Discarded, .type_id = expr.type_id()});
 
   // TODO: This will eventually need to do some "do not discard" analysis.
 }
 
-auto HandleExprStatement(Context& context,
-                         Parse::ExprStatementId /*parse_node*/) -> bool {
+auto HandleExprStatement(Context& context, Parse::ExprStatementId /*node_id*/)
+    -> bool {
   HandleDiscardedExpr(context, context.node_stack().PopExpr());
   return true;
 }