feat: give column data types a dedicated syntax-highlight colour

Both Node::Ident and Word carried a highlight_override field, and
both were dead — the walker driver discarded the Ident's and
walk_word hardcoded Keyword. So column types (int, serial, …)
rendered identically to table/column names.

Wire both overrides through, and add a dedicated HighlightClass::Type
with its own theme colour (tok_type), distinct from keyword-purple
and identifier-teal. The three type Ident slots opt in, so canonical
types and the advanced-mode single-word SQL aliases (float, varchar,
…) render as types; the two-word `double precision` alias opts in via
a new Word::type_keyword constructor. ADR-0022 Amendment 4.

src/dsl/grammar/shared.rs

@@ -7,8 +7,8 @@
 
 use crate::completion::TableColumn;
 use crate::dsl::grammar::{
-    HintMode, IdentSource, IdentValidator, Node, NumberValidator,
-    ValidationError, Word,
+    HighlightClass, HintMode, IdentSource, IdentValidator, Node,
+    NumberValidator, ValidationError, Word,
 };
 use crate::dsl::types::Type;
 use crate::dsl::walker::context::WalkContext;
@@ -50,7 +50,7 @@ pub const TYPE_SLOT: Node = Node::Ident {
     source: IdentSource::Types,
     role: "type",
     validator: Some(TYPE_VALIDATOR),
-    highlight_override: None,
+    highlight_override: Some(HighlightClass::Type),
         writes_table: false,
         writes_column: false,
         writes_user_listed_column: false,