feat(lsp): highlight semantic token modifiers (#21390)

Apply semantic token modifiers as separate extmarks with corresponding
highlight groups (e.g., `@readonly`). This is a low-effort PR to enable
the most common use cases (applying, e.g., italics or backgrounds on top
of type highlights; language-specific fallbacks like `@global.lua` are
also available). This can be replaced by more complicated selector-style
themes later on.

runtime/doc/lsp.txt

@@ -1340,7 +1340,7 @@ start({bufnr}, {client_id}, {opts})          *vim.lsp.semantic_tokens.start()*
     |vim.lsp.buf_attach_client()|. To opt-out of semantic highlighting with a
     server that supports it, you can delete the semanticTokensProvider table
     from the {server_capabilities} of your client in your |LspAttach| callback
-    or your configuration's `on_attach` callback. >lua
+    or your configuration's `on_attach` callback: >lua
 
        client.server_capabilities.semanticTokensProvider = nil
 <

runtime/lua/vim/lsp/semantic_tokens.lua

@@ -409,7 +409,17 @@ function STHighlighter:on_win(topline, botline)
             strict = false,
           })
 
-          --TODO(jdrouhard): do something with the modifiers
+          -- TODO(bfredl) use single extmark when hl_group supports table
+          if #token.modifiers > 0 then
+            for _, modifier in pairs(token.modifiers) do
+              api.nvim_buf_set_extmark(self.bufnr, state.namespace, token.line, token.start_col, {
+                hl_group = '@' .. modifier,
+                end_col = token.end_col,
+                priority = vim.highlight.priorities.semantic_tokens,
+                strict = false,
+              })
+            end
+          end
 
           token.extmark_added = true
         end
@@ -494,7 +504,7 @@ local M = {}
 --- opt-out of semantic highlighting with a server that supports it, you can
 --- delete the semanticTokensProvider table from the {server_capabilities} of
 --- your client in your |LspAttach| callback or your configuration's
---- `on_attach` callback.
+--- `on_attach` callback:
 --- <pre>lua
 ---   client.server_capabilities.semanticTokensProvider = nil
 --- </pre>