fix: only create foreground when inversing highlight group

## Details

Issue: #154

When creating an inverse highlight group for borders we currently swap
both the foreground and background. However there's never really a need
to do this since all we care about is applying the original background
as the new foreground.

Be removing the background property we should avoid issues where users
set a foreground as well but expect the result to blend in with the
overall background.

This also more closely mirrors the goal of the logic.

Author: MeanderingProgrammer

Diff for: doc/render-markdown.txt

@@ -1,4 +1,4 @@
-*render-markdown.txt*         For 0.10.0        Last change: 2024 September 01
+*render-markdown.txt*         For 0.10.0        Last change: 2024 September 08
 
 ==============================================================================
 Table of Contents                          *render-markdown-table-of-contents*

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

@@ -80,15 +80,12 @@ end
 
 ---@param highlight string
 ---@return string
-function M.inverse(highlight)
+function M.inverse_bg(highlight)
     local name = string.format('%s_Inverse_%s', M.prefix, highlight)
     if not vim.tbl_contains(cache, name) then
         local hl = M.get_hl(highlight)
         vim.api.nvim_set_hl(0, name, {
             fg = hl.bg,
-            bg = hl.fg,
-            ---@diagnostic disable-next-line: undefined-field
-            ctermbg = hl.ctermfg,
             ---@diagnostic disable-next-line: undefined-field
             ctermfg = hl.ctermbg,
         })

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

@@ -5,7 +5,7 @@ local M = {}
 
 ---@private
 ---@type string
-M.version = '6.3.2'
+M.version = '6.3.3'
 
 function M.check()
     vim.health.start('render-markdown.nvim [version]')

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

@@ -139,14 +139,14 @@ function Render:background(icon_added)
         local border_width = width - self.data.col
         if not icon_added and self.context:hidden(self.data.code_info) and self:delim_hidden(self.data.start_row) then
             self.marks:add(true, self.data.start_row, self.data.col, {
-                virt_text = { { self.code.above:rep(border_width), colors.inverse(self.code.highlight) } },
+                virt_text = { { self.code.above:rep(border_width), colors.inverse_bg(self.code.highlight) } },
                 virt_text_pos = 'overlay',
             })
             self.data.start_row = self.data.start_row + 1
         end
         if self:delim_hidden(self.data.end_row - 1) then
             self.marks:add(true, self.data.end_row - 1, self.data.col, {
-                virt_text = { { self.code.below:rep(border_width), colors.inverse(self.code.highlight) } },
+                virt_text = { { self.code.below:rep(border_width), colors.inverse_bg(self.code.highlight) } },
                 virt_text_pos = 'overlay',
             })
             self.data.end_row = self.data.end_row - 1

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

@@ -166,7 +166,7 @@ function Render:border(width)
         return
     end
 
-    local background = colors.inverse(self.data.background)
+    local background = colors.inverse_bg(self.data.background)
     local prefix = self.heading.border_prefix and self.data.level or 0
 
     local line_above = {