chore!: Complete deprecation of custom handlers render method

## Details

Completes the removal of `render` method in `custom_handlers`.

Originally slated for 2024-08-19, I'll be out around this time for a few
days and want to get this plugin into a steady state after the rename.

Get the lua rock updated figure out what to do with the old one, etc.

I did double check the feature is unused, at least from what's available on Github:

- https://github.com/search?q=custom_handlers+render+++language%3ALua&type=code
- https://github.com/search?q=custom_handlers+markdown++language%3ALua&type=code
- https://github.com/search?q=custom_handlers+MeanderingProgrammer++language%3ALua&type=code

No one appears to be using it which gives me enough confidence to remove it early

Author: MeanderingProgrammer

Diff for: CHANGELOG.md

@@ -1,6 +1,8 @@
 # Change Log
 
-## Pre-release (slated for 2024-08-19)
+## Pre-release
+
+## 6.0.0 (2024-08-05)
 
 ### ⚠ BREAKING CHANGES
 

Diff for: benches/readme_spec.lua

@@ -4,7 +4,7 @@ local util = require('benches.util')
 
 describe('README.md', function()
     it('default', function()
-        local base_marks = 55
+        local base_marks = 54
         util.less_than(util.setup('README.md'), 50)
         util.num_marks(base_marks)
 

Diff for: doc/todo.md

@@ -4,4 +4,3 @@
   alignment info.
 - Figure out how to display the many configuration options & impact, Wiki in progress
 - Potentially change LuaRocks icon dependency to [mini.icons](https://luarocks.org/modules/neorocks/mini.icons)
-- Rename plugin refernces in relevant places, fun :[

Diff for: lua/render-markdown/ui.lua

@@ -159,29 +159,18 @@ function M.parse_tree(buf, language, root)
     end
 
     local marks = {}
-    local user_handler = state.custom_handlers[language]
-    if user_handler ~= nil then
+    local user = state.custom_handlers[language]
+    if user ~= nil then
         logger.debug('running handler', 'user')
-        -- TODO: remove call to render & parse nil check
-        ---@diagnostic disable-next-line: undefined-field
-        if user_handler.render ~= nil then
-            local message = 'render-markdown.nvim: custom_handlers render is deprecated use parse instead'
-            message = message .. ', will be fully removed on 2024-08-19'
-            vim.notify_once(message, vim.log.levels.ERROR)
-            ---@diagnostic disable-next-line: undefined-field
-            user_handler.render(M.namespace, root, buf)
-        end
-        if user_handler.parse ~= nil then
-            vim.list_extend(marks, user_handler.parse(root, buf))
-        end
-        if not user_handler.extends then
+        vim.list_extend(marks, user.parse(root, buf))
+        if not user.extends then
             return marks
         end
     end
-    local builtin_handler = builtin_handlers[language]
-    if builtin_handler ~= nil then
+    local builtin = builtin_handlers[language]
+    if builtin ~= nil then
         logger.debug('running handler', 'builtin')
-        vim.list_extend(marks, builtin_handler.parse(root, buf))
+        vim.list_extend(marks, builtin.parse(root, buf))
     end
     return marks
 end

Diff for: tests/custom_handler_spec.lua

@@ -52,7 +52,7 @@ describe('custom_handler.md', function()
         util.marks_are_equal(expected, actual)
     end)
 
-    it('custom override parse', function()
+    it('custom parse override', function()
         util.setup('tests/data/custom_handler.md', {
             custom_handlers = {
                 markdown_inline = {
@@ -73,7 +73,7 @@ describe('custom_handler.md', function()
         util.marks_are_equal(expected, actual)
     end)
 
-    it('custom extends', function()
+    it('custom parse extend', function()
         util.setup('tests/data/custom_handler.md', {
             custom_handlers = {
                 markdown_inline = {