feat(lsp): add references provider for goto references

Add referencesProvider capability and handler to support LSP
references (gr keybinding in LazyVim). Searches all occurrences
of a symbol in the current document.

src/main.lua

@@ -61,6 +61,7 @@ local capabilities = {
 		resolveProvider = false,
 	},
 	definitionProvider = true,
+	referencesProvider = true,
 	documentSymbolProvider = true,
 	diagnosticProvider = { relatedDocuments = {} },
 	textDocumentSync = 1,
@@ -456,6 +457,67 @@ function handlers.textDocument_definition(params)
 	return nil
 end
 
+function handlers.textDocument_references(params)
+	local uri = params.textDocument.uri
+	local doc = documents[uri]
+	if not doc then
+		return nil
+	end
+
+	local lines = {}
+	for line in doc.content:gmatch("([^\n]*)\n") do
+		table.insert(lines, line)
+	end
+	if params.position.line + 1 > #lines then
+		return nil
+	end
+
+	local line = lines[params.position.line + 1]
+	local col = params.position.character + 1
+
+	local word_start = col
+	while word_start > 1 and line:sub(word_start - 1, word_start - 1):match("[%w_]") do
+		word_start = word_start - 1
+	end
+
+	local word_end = col
+	while word_end <= #line and line:sub(word_end, word_end):match("[%w_]") do
+		word_end = word_end + 1
+	end
+
+	local word = line:sub(word_start, word_end - 1):gsub("^#", "")
+	if not word or word == "" then
+		return nil
+	end
+
+	local references = {}
+	for line_num, line_content in ipairs(lines) do
+		local search_col = 1
+		while search_col <= #line_content do
+			local match_start, match_end = line_content:find("[%w_]+", search_col)
+			if not match_start then
+				break
+			end
+			local matched_word = line_content:sub(match_start, match_end)
+			if matched_word == word then
+				table.insert(references, {
+					uri = uri,
+					range = {
+						start = { line = line_num - 1, character = match_start - 1 },
+						["end"] = { line = line_num - 1, character = match_end },
+					},
+				})
+			end
+			search_col = match_end + 1
+		end
+	end
+
+	if #references > 0 then
+		return references
+	end
+	return nil
+end
+
 function handlers.textDocument_documentSymbol(params)
 	local uri = params.textDocument.uri
 	local doc = documents[uri]