Syntactic highlighting of NAME_REF for injections

This commit adds a function that tries to determine the syntax highlighting class of NAME_REFs based on the usage.
It is used for highlighting injections (such as highlighting of doctests) as the semantic logic will most of the time result in unresolved references.
It also adds a color to unresolved references in HTML encoding.

crates/ra_ide/src/syntax_highlighting/injection.rs

@@ -137,7 +137,7 @@ pub(super) fn highlight_doc_comment(
     let (analysis, tmp_file_id) = Analysis::from_single_file(text);
 
     stack.push();
-    for mut h in analysis.highlight(tmp_file_id).unwrap() {
+    for mut h in analysis.with_db(|db| super::highlight(db, tmp_file_id, None, true)).unwrap() {
         // Determine start offset and end offset in case of multi-line ranges
         let mut start_offset = None;
         let mut end_offset = None;