nested blocks

[justinmk]

(Continuation of #13 . See also #21 )

Problem

#16 added (block), but these don't "nest".

Preamble from #12 :

In order for :help to render as something other than fixed-width, hard-wrapped text, (e.g. for HTML format) the parser needs to give some basic hints about structure besides "line". Sometimes (often) the :help source needs to be fixed (because it can't be reasonably parsed), but in other common cases it's probably reasonable for tree-sitter-vimdoc to provide some basic idea of:

Example of nested (indented) blocks

https://github.com/neovim/neovim/blob/52e4b779e3df1335efc4774e8dfc199d07e24bfd/runtime/doc/eval.txt#L51-L74

"Nested blocks" simply means blocks of text that have common indentation. In the following example,

• we can't parse the Number 123... block but if we can at least recognize that it is indented, then the HTML result can wrap that block in <pre> so that it is readable.
• the *octal* line doesn't share indentation so it should be parsed as (line) (or possibly introduce a new (tags_heading) node?).
• it may be reasonable for us to require blank lines between these blocks.

The Number and String types are converted automatically, depending on how they
are used.

Conversion from a Number to a String is by making the ASCII representation of
the Number.  Examples:
	Number 123	-->	String "123" ~
	Number 0	-->	String "0" ~
	Number -1	-->	String "-1" ~
							*octal*
Conversion from a String to a Number is done by converting the first digits to
a number.  Hexadecimal "0xf9", Octal "017" or "0o17", and Binary "0b10"
numbers are recognized.  If the String doesn't start with digits, the result
is zero. Examples:
	String "456"	-->	Number 456 ~
	String "6bar"	-->	Number 6 ~
	String "foo"	-->	Number 0 ~
	String "0xf1"	-->	Number 241 ~
	String "0100"	-->	Number 64 ~
	String "0o100"	-->	Number 64 ~
	String "0b101"	-->	Number 5 ~
	String "-8"	-->	Number -8 ~
	String "+8"	-->	Number 0 ~

To force conversion from String to Number, add zero to it: >
	:echo "0100" + 0
<	64 ~

To avoid a leading zero to cause octal conversion, or for using a different
base, use |str2nr()|.