refactor: improve formula parser

[Myriad-Dreamin]

I have refactored the parser and added some tests for formulas.

During testing, I find a bug:

mitex/crates/mitex/src/lib.rs

Lines 1151 to 1156 in b85675a

	// todo: fix this bug
	assert_debug_snapshot!(convert_text(r#"$ a $"#), @r###"
	Ok(
	"$ a $",
	)
	"###);

And two optional semantic errors to handle:

mitex/crates/mitex/src/lib.rs

Lines 1157 to 1168 in b85675a

	// todo: fix semantic incorrect cases
	assert_debug_snapshot!(convert_text(r#"$$ a $ $ b $$"#), @r###"
	Ok(
	"$ a $ $ b $",
	)
	"###);
	// todo: fix semantic incorrect cases
	assert_debug_snapshot!(convert_text(r#"\[a\)\(b\]"#), @r###"
	Ok(
	"$ a $$b $",
	)
	"###);

[OrangeX4]

Now, in text mode, we translate $ a $ as #math.equation(block: false, $ a $); instead of using bare equation, to address the first bug. However, I'm not quite sure about the next two optional semantic errors, but they seem to be just edge cases. Should we not prioritize them for now?

[Myriad-Dreamin]

Now, in text mode, we translate $ a $ as #math.equation(block: false, $ a $); instead of using bare equation, to address the first bug.

Sounds great before we having taken all whitespace into control.

However, I'm not quite sure about the next two optional semantic errors, but they seem to be just edge cases. Should we not prioritize them for now?

You can do it now, but it is not urgent since they don't occur in regular document. In addition, this is not introduced by this PR and has stayed for a long time.

[OrangeX4]

Ok, I've added formula check.