feat: add a default icon for indent feature

## Details

This is a quick follow up to add a default icon to the indent feature:
21623a9

I think this feature makes sense to push out to users and have them
opt-out rather than discovering it.

To opt-out set the `icon` value to the empty string:

```lua
require('render-markdown').setup({
    indent = {
        enabled = true,
        icon = '',
    },
})
```

Author: MeanderingProgrammer

README.md

@@ -681,7 +681,7 @@ require('render-markdown').setup({
         -- Do not indent heading titles, only the body
         skip_heading = false,
         -- Prefix added when indenting, one per level
-        icon = '',
+        icon = '▎',
         -- Applied to icon
         highlight = 'RenderMarkdownIndent',
     },
@@ -1341,7 +1341,7 @@ require('render-markdown').setup({
         -- Do not indent heading titles, only the body
         skip_heading = false,
         -- Prefix added when indenting, one per level
-        icon = '',
+        icon = '▎',
         -- Applied to icon
         highlight = 'RenderMarkdownIndent',
     },

doc/render-markdown.txt

@@ -742,7 +742,7 @@ Default Configuration ~
             -- Do not indent heading titles, only the body
             skip_heading = false,
             -- Prefix added when indenting, one per level
-            icon = '',
+            icon = '▎',
             -- Applied to icon
             highlight = 'RenderMarkdownIndent',
         },
@@ -1378,7 +1378,7 @@ Indent Configuration ~
             -- Do not indent heading titles, only the body
             skip_heading = false,
             -- Prefix added when indenting, one per level
-            icon = '',
+            icon = '▎',
             -- Applied to icon
             highlight = 'RenderMarkdownIndent',
         },

lua/render-markdown/health.lua

@@ -5,7 +5,7 @@ local state = require('render-markdown.state')
 local M = {}
 
 ---@private
-M.version = '8.0.9'
+M.version = '8.0.10'
 
 function M.check()
     M.start('version')

lua/render-markdown/init.lua

@@ -817,7 +817,7 @@ M.default_config = {
         -- Do not indent heading titles, only the body
         skip_heading = false,
         -- Prefix added when indenting, one per level
-        icon = '',
+        icon = '▎',
         -- Applied to icon
         highlight = 'RenderMarkdownIndent',
     },

tests/indent_spec.lua

@@ -70,10 +70,10 @@ local function indent_mark(mark, lengths)
 end
 
 describe('indent.md', function()
-    it('with heading border', function()
+    it('with heading border & no icon', function()
         util.setup('tests/data/indent.md', {
             heading = { border = true },
-            indent = { enabled = true },
+            indent = { enabled = true, icon = '' },
         })
 
         local expected, row = {}, util.row()
@@ -131,9 +131,9 @@ describe('indent.md', function()
         })
     end)
 
-    it('with per_level, skip_level & icon', function()
+    it('with per_level & skip_level', function()
         util.setup('tests/data/indent.md', {
-            indent = { enabled = true, per_level = 4, skip_level = 0, icon = '▎' },
+            indent = { enabled = true, per_level = 4, skip_level = 0 },
         })
 
         local expected, row = {}, util.row()