Use strdisplaywidth in place of # for string lengths

Resolves: #26

Currently there are a couple places where when calculating the
length of a particular string we simply use the Lua `#`. This works
well enough for most characters, but not well with more complex ones
such as with Chinese, as described in the issue.

Using strdisplaywidth accounts for how Neovim will actually render the
characters.

Author: MeanderingProgrammer

lua/render-markdown/handler/markdown.lua

@@ -17,7 +17,7 @@ M.render = function(namespace, root, buf)
         logger.debug_node(capture, node, buf)
 
         if capture == 'heading' then
-            local level = #value
+            local level = vim.fn.strdisplaywidth(value)
 
             local heading = list.cycle(state.config.headings, level)
             -- Available width is level + 1, where level = number of `#` characters and one is added
@@ -54,7 +54,7 @@ M.render = function(namespace, root, buf)
             local list_marker_overlay = ''
             if M.is_sibling_checkbox(node) then
                 -- Hide the list marker for checkboxes rather than replacing with a bullet point
-                list_marker_overlay = string.rep(' ', #value)
+                list_marker_overlay = string.rep(' ', vim.fn.strdisplaywidth(value))
             else
                 -- List markers from tree-sitter should have leading spaces removed, however there are known
                 -- edge cases in the parser: https://github.com/tree-sitter-grammars/tree-sitter-markdown/issues/127
@@ -103,10 +103,10 @@ M.render = function(namespace, root, buf)
                 local lines = vim.api.nvim_buf_get_lines(buf, start_row, end_row, false)
                 local table_head = list.first(lines)
                 local table_tail = list.last(lines)
-                if #table_head == #table_tail then
+                if vim.fn.strdisplaywidth(table_head) == vim.fn.strdisplaywidth(table_tail) then
                     local headings = vim.split(table_head, '|', { plain = true, trimempty = true })
                     local sections = vim.tbl_map(function(part)
-                        return string.rep('─', #part)
+                        return string.rep('─', vim.fn.strdisplaywidth(part))
                     end, headings)
 
                     local line_above = { { '┌' .. table.concat(sections, '┬') .. '┐', highlights.table.head } }