Remove theme parameters from buffer/display map's chunks methods

Change Chunks to contain highlight ids instead of actual highlight
styles. Retrieve the actual highlight style from the theme in the
editor element layer.

This is to set us up to perform syntax highlighting in other code
paths where the theme is not available.

crates/language/src/tests.rs

@@ -1090,7 +1090,7 @@ fn chunks_with_diagnostics<T: ToOffset + ToPoint>(
     range: Range<T>,
 ) -> Vec<(String, Option<DiagnosticSeverity>)> {
     let mut chunks: Vec<(String, Option<DiagnosticSeverity>)> = Vec::new();
-    for chunk in buffer.snapshot().chunks(range, Some(&Default::default())) {
+    for chunk in buffer.snapshot().chunks(range) {
         if chunks
             .last()
             .map_or(false, |prev_chunk| prev_chunk.1 == chunk.diagnostic)