chore(types): add extension type for vim.api.keyset.set_extmark

## Details

Rather than working with opts type `vim.api.keyset.set_extmark`
directly, add an extension type `render.md.MarkOpts` which provides
more strict type definitions for:

- `virt_text`: `any[]` -> `render.md.MarkLine`
- `virt_text_pos`: `string` -> `'eol'|'inline'|'overlay'`
- `virt_lines`: `any[]` -> `render.md.MarkLine[]`

Minor other changes:

- Remove `public` scope from fields
- Rename `render.md.Text` -> `render.md.MarkText`
- Rename `render.md.Line` -> `render.md.MarkLine`

Author: MeanderingProgrammer

doc/custom-handlers.md

@@ -12,19 +12,30 @@ needed, and concealing when the cursor enters.
 Each handler must conform to the following interface:
 
 ```lua
+---@class (exact) render.md.MarkText
+---@field [1] string text
+---@field [2] string|string[] highlights
+
+---@alias render.md.MarkLine render.md.MarkText[]
+
+---@class render.md.MarkOpts: vim.api.keyset.set_extmark
+---@field virt_text? render.md.MarkLine
+---@field virt_text_pos? 'eol'|'inline'|'overlay'
+---@field virt_lines? render.md.MarkLine[]
+
 ---@class (exact) render.md.Mark
----@field public conceal boolean
----@field public start_row integer
----@field public start_col integer
----@field public opts vim.api.keyset.set_extmark
+---@field conceal boolean
+---@field start_row integer
+---@field start_col integer
+---@field opts render.md.MarkOpts
 
 ---@class (exact) render.md.HandlerContext
----@field public buf integer
----@field public root TSNode
+---@field buf integer
+---@field root TSNode
 
 ---@class (exact) render.md.Handler
----@field public extends? boolean
----@field public parse fun(ctx: render.md.HandlerContext): render.md.Mark[]
+---@field extends? boolean
+---@field parse fun(ctx: render.md.HandlerContext): render.md.Mark[]
 ```
 
 The `parse` function takes a `ctx` parameter whose fields are:

doc/render-markdown.txt

@@ -1,4 +1,4 @@
-*render-markdown.txt*           For 0.10.0          Last change: 2025 March 09
+*render-markdown.txt*           For 0.10.0          Last change: 2025 March 10
 
 ==============================================================================
 Table of Contents                          *render-markdown-table-of-contents*

lua/render-markdown/debug/marks.lua

@@ -2,7 +2,7 @@ local Iter = require('render-markdown.lib.iter')
 
 ---@class render.md.debug.Mark
 ---@field conceal boolean
----@field opts vim.api.keyset.set_extmark
+---@field opts render.md.MarkOpts
 ---@field row { [1]: integer, [2]: integer }
 ---@field col { [1]: integer, [2]: integer }
 local Mark = {}

lua/render-markdown/health.lua

@@ -5,7 +5,7 @@ local state = require('render-markdown.state')
 local M = {}
 
 ---@private
-M.version = '8.1.1'
+M.version = '8.1.2'
 
 function M.check()
     M.start('version')