Draft: ignore include in a code block

[ejiektpobehuk]

Addresses #717

Before the change:

• processes all Include directives, even those inside code blocks

After the change:

• finds all fenced code block positions
• finds all Include directive positions
• processes only Include directives that are outside of fenced code blocks

---

I'm not well versed in go so I took the liberty to generate most of the code and tests for this PR by opus 4.5.

They passed my initial sniff test so I'm posting it publicly.
The PR is marked as a draft since I want to take more responsibility for the change:

• get more familiar with the code
• test it in production
• fix code blocks with the syntax highlight
• make tests more readable
• support for indented code block.

---

Mermaid tests were failing before the change in the exact same manner:

go test ./...
ok  	github.com/kovetskiy/mark	0.016s
ok  	github.com/kovetskiy/mark/attachment	0.003s
?   	github.com/kovetskiy/mark/confluence	[no test files]
ok  	github.com/kovetskiy/mark/d2	0.624s
?   	github.com/kovetskiy/mark/includes	[no test files]
?   	github.com/kovetskiy/mark/macro	[no test files]
ok  	github.com/kovetskiy/mark/markdown	0.043s
--- FAIL: TestExtractMermaidImage (0.62s)
    --- FAIL: TestExtractMermaidImage/example (0.62s)
        mermaid_test.go:45:
            	Error Trace:	/tmp/mark/mermaid/mermaid_test.go:45
            	Error:      	Not equal:
            	            	expected: "87"
            	            	actual  : "86"
            	            	
            	            	Diff:
            	            	--- Expected
            	            	+++ Actual
            	            	@@ -1 +1 @@
            	            	-87
            	            	+86
            	Test:       	TestExtractMermaidImage/example
            	Messages:   	processMermaidLocally(example, graph TD;
            	            	 A-->B;)
FAIL
FAIL	github.com/kovetskiy/mark/mermaid	0.629s
ok  	github.com/kovetskiy/mark/metadata	0.003s
ok  	github.com/kovetskiy/mark/page	0.003s
?   	github.com/kovetskiy/mark/parser	[no test files]
?   	github.com/kovetskiy/mark/renderer	[no test files]
?   	github.com/kovetskiy/mark/stdlib	[no test files]
?   	github.com/kovetskiy/mark/types	[no test files]
ok  	github.com/kovetskiy/mark/util	0.016s
?   	github.com/kovetskiy/mark/vfs	[no test files]
FAIL