Shift Parser from 'Identifier' to 'Name' naming (#2947)

This PR renames parse nodes on a Name/NameExpression taxonomy. NameExpressions occur in a name context. The difference is that in non-expression contexts it's useful to return the identifier / string ID for adding to name lookup, whereas in expression contexts it's useful to return the resolved node ID for consistency with other expressions.

In the code, I do note SelfValueName is returned in the expression context: I'd expect this to change, as `self` in `[self: Self]` versus `self.Foo()` will probably be best handled similarly to the above. That means that, in the proposed taxonomy, both `SelfValueName` and `SelfValueNameExpression` will exist in order to assist semantics.

To contrast choices:

Original | Current | [zygoloid suggestion](https://discord.com/channels/655572317891461132/655578254970716160/1121581663399464970) | [This PR](https://discord.com/channels/655572317891461132/655578254970716160/1121814551789318215)
--- | --- | --- | ---
DeclaredName/DesignatedName | Identifier | NameComponent | Name
NameReference | NameReference | NameReference | NameExpression
SelfValueIdentifier | SelfValueIdentifier | SelfValueReference | SelfValueName
SelfTypeIdentifier | SelfTypeIdentiifer | SelfTypeReference | SelfTypeNameExpression

toolchain/semantics/semantics_handle_function.cpp

@@ -53,7 +53,7 @@ auto SemanticsHandleFunctionDefinitionStart(SemanticsContext& context,
       ParseNodeKind::ParameterList);
   auto [name_node, name_id] =
       context.node_stack().PopWithParseNode<SemanticsStringId>(
-          ParseNodeKind::Identifier);
+          ParseNodeKind::Name);
   auto fn_node = context.node_stack().PopForSoloParseNode(
       ParseNodeKind::FunctionIntroducer);