feat: added support for custom parser for markdown to document

[Mukund-Tandon]

fixes: #401

[LucasXu0]

@Mukund-Tandon Please format the code

[Mukund-Tandon]

@Mukund-Tandon Please format the code

Ok,I am also writing test for it

[codecov]

Codecov Report

Merging #403 (5a08ee9) into main (56474e8) will decrease coverage by 0.16%.
Report is 14 commits behind head on main.
The diff coverage is 89.28%.

@@            Coverage Diff             @@
##             main     #403      +/-   ##
==========================================
- Coverage   80.75%   80.60%   -0.16%     
==========================================
  Files         252      254       +2     
  Lines       10258    10393     +135     
==========================================
+ Hits         8284     8377      +93     
- Misses       1974     2016      +42     

Files Changed	Coverage Δ
...ns/markdown/decoder/document_markdown_decoder.dart	93.57% <82.35%> (-1.32%)	⬇️
...ugins/markdown/decoder/delta_markdown_decoder.dart	100.00% <100.00%> (ø)
lib/src/plugins/markdown/document_markdown.dart	100.00% <100.00%> (ø)

... and 21 files with indirect coverage changes

[LucasXu0]

Suggested change

	for (final key in element.attributes.keys) {
	if (element.attributes[key] != null) {
	_attributes[key] = jsonDecode(element.attributes[key]!);
	}
	}
	element.attributes.forEach((key, value) {
	_attributes[key] = jsonDecode(value);
	});

[LucasXu0]

Suggested change

	for (var key in element.attributes.keys) {
	for (final key in element.attributes.keys) {

[LucasXu0]

Suggested change

	document
	.insert([i], [_convertLineToNode(lines[i], customInlineSyntaxes)]);
	document.insert(
	[i],
	[_convertLineToNode(lines[i], customInlineSyntaxes)],
	);

[LucasXu0]

Suggested change

	document
	.insert([i], [_convertLineToNode(lines[i], customInlineSyntaxes)]);
	document.insert(
	[i],
	[_convertLineToNode(lines[i], customInlineSyntaxes)],
	);

[LucasXu0]

Suggested change

	final decoder =
	DeltaMarkdownDecoder(customInlineSyntaxes: customInlineSyntaxes);
	for (final parser in customNodeParsers) {
	final Node? node = parser.transform(line);
	if (node != null) {
	return node;
	}
	}
	final decoder = DeltaMarkdownDecoder(
	customInlineSyntaxes: customInlineSyntaxes,
	);
	for (final parser in customNodeParsers) {
	final node = parser.transform(line);
	if (node != null) {
	return node;
	}
	}

[LucasXu0]

@Mukund-Tandon I refactored some codes. The suggestions are just for your reference, don't commit them.

[Mukund-Tandon]

@Mukund-Tandon I refactored some codes. The suggestions are just for your reference, don't commit them.

Ok 👍