[banjo] Add support for string constants.

We were parsing string constants as identifiers instead of strings.

TODO: Add test with string constant.
Change-Id: Ia493c4afbbf87eebe2447e3dc0aa8783d6baa9f3

tools/banjo/banjo/src/ast.rs

@@ -787,7 +787,7 @@ impl BanjoAst {
                         Rule::ident => {
                             name = String::from(inner_pair.as_str());
                         }
-                        Rule::identifier_type | Rule::primitive_type => {
+                        Rule::identifier_type | Rule::string_type | Rule::primitive_type => {
                             ty = Ty::from_pair(ns, &inner_pair)?;
                         }
                         Rule::constant => {

tools/banjo/banjo/src/parser.rs

@@ -22,7 +22,7 @@ declaration_list = _{ ( declaration ~ ";" )* }
 
 declaration = _{ const_declaration | enum_declaration | union_declaration | struct_declaration | interface_declaration }
 
-const_declaration = { attributes ~ "const" ~ ( primitive_type | identifier_type ) ~ ident ~ "=" ~ constant }
+const_declaration = { attributes ~ "const" ~ ( primitive_type | string_type | identifier_type ) ~ ident ~ "=" ~ constant }
 
 doc_comment = ${ "///" ~ (" " | SYMBOL | PUNCTUATION | ASCII_ALPHANUMERIC)* ~ "\n"}
 doc_comment_block = _{ (doc_comment)* }