feat: add virtual space nodes and improve linebreak positioning (#1)

- virtual spaces are spaces that were expanded from tabs
- virutal spaces are represented in the syntax tree only if they should be rendered in the HTML output
- the idea of virtual spaces is borrowed from [`micromark/common-markup-state-machine`](https://github.com/micromark/common-markup-state-machine#cvs)
- linebreaks in indented code block, fenced code block, and html blocks are now more accurate so that we could grab their value contents more easily

corpus/custom.txt

@@ -592,6 +592,40 @@ is-autolink     www.su_b.example.com
     (soft_line_break)
     (www_autolink)))
 
+================================================================================
+Virtual space
+================================================================================
+   -    	three virtual spaces
+
+- ```
+	two virtual spaces
+  ```
+
+-  <script>
+	/* one virtual space */</script>
+
+--------------------------------------------------------------------------------
+
+(document
+  (loose_list
+    (list_item
+      (list_marker)
+      (indented_code_block
+        (virtual_space)
+        (virtual_space)
+        (virtual_space)))
+    (list_item
+      (list_marker)
+      (fenced_code_block
+        (virtual_space)
+        (virtual_space)
+        (code_fence_content)))
+    (list_item
+      (list_marker)
+      (html_block
+        (line_break)
+        (virtual_space)))))
+
 ================================================================================
 Unnamed 1
 ================================================================================

corpus/spec.txt

@@ -60,7 +60,9 @@ Example 5 - https://github.github.com/gfm/#example-5
     (list_item
       (list_marker)
       (paragraph)
-      (indented_code_block))))
+      (indented_code_block
+        (virtual_space)
+        (virtual_space)))))
 
 ================================================================================
 Example 6 - https://github.github.com/gfm/#example-6
@@ -71,7 +73,9 @@ Example 6 - https://github.github.com/gfm/#example-6
 
 (document
   (block_quote
-    (indented_code_block)))
+    (indented_code_block
+      (virtual_space)
+      (virtual_space))))
 
 ================================================================================
 Example 7 - https://github.github.com/gfm/#example-7
@@ -84,7 +88,9 @@ Example 7 - https://github.github.com/gfm/#example-7
   (tight_list
     (list_item
       (list_marker)
-      (indented_code_block))))
+      (indented_code_block
+        (virtual_space)
+        (virtual_space)))))
 
 ================================================================================
 Example 8 - https://github.github.com/gfm/#example-8

grammar.js

@@ -10,6 +10,7 @@ module.exports = grammar({
                                                 $._lnk_ref_def_end_mkr,
     $._stx_bgn,                                 $._stx_end_mkr,
     $._atx_bgn,                                 $._atx_end_mkr,
+    $._ind_cod_bgn_pfx,
     $._ind_cod_bgn_mkr,                         $._ind_cod_end_mkr,
     $._fen_cod_bgn,         $._fen_cod_end,     $._fen_cod_end_mkr,
     $._htm_blk_scr_bgn,     $._htm_blk_scr_end, $._htm_blk_scr_end_mkr,
@@ -68,13 +69,15 @@ module.exports = grammar({
     $._bnk_lbk,
     $._lit_lbk,
     $._txt,
+
+    $.virtual_space,
   ],
 
   conflicts: $ => [
     [$._chk_box_pgh_ctn, $._chk_box_lik_stx_ctn], // requires 2 lookahead tokens to distinguish them
   ],
 
-  extras: $ => [$._lka, $._lit_lbk],
+  extras: $ => [$._lka, $._lit_lbk, $.virtual_space],
 
   rules: {
     document: $ => seq(repeat(choice($._blk_nod, $._bnk_lbk)), $._eof),
@@ -91,7 +94,7 @@ module.exports = grammar({
     _stx_hed: $ => seq(alias($._pgh_hed, $.heading_content), $._pgh_end_mkr, $._stx_bgn),
     _atx: $ => seq($._atx_hed, $._atx_end_mkr),
     _atx_hed: $ => seq($._atx_bgn, alias(repeat($._inl_nod), $.heading_content), optional($._atx_end)),
-    _ind_cod: $ => seq($._ind_cod_hed, $._ind_cod_end_mkr),
+    _ind_cod: $ => seq($._ind_cod_bgn_pfx, $._ind_cod_hed, $._ind_cod_end_mkr),
     _ind_cod_hed: $ => seq($._ind_cod_bgn_mkr, repeat(choice($._txt, $._blk_lbk))),
     _fen_cod: $ => seq($._fen_cod_hed, $._fen_cod_end_mkr),
     _fen_cod_hed: $ => seq($._fen_cod_bgn, optional(seq($._fen_cod_inf_bgn_mkr, alias(repeat(choice($._inl_txt)), $.info_string), $._fen_cod_inf_end_mkr)), optional(seq($._blk_lbk, alias(repeat(choice($._txt, alias($._blk_lbk, $.line_break))), $.code_fence_content))), optional(seq($._blk_lbk, $._fen_cod_end))),