Markdown autolink resolution _is_mailto can fail with bad string indexing

[tlienart]

This was reported by a Franklin user but ended up being a bug with the Markdown module:

julia> Markdown.parse("<一轮红日初升>")
ERROR: StringIndexError: invalid index [6], valid nearby indices [4]=>'轮', [7]=>'红'

This is because Markdown sees the < ... > and tries to form an auto link and seems to be using string indexing in doing so. CommonMark.jl handles this well

julia> import CommonMark as CM
julia> p = CM.Parser()
julia> CM.html(p("<一轮红日初升>"))
"<p>&lt;一轮红日初升&gt;</p>\n"

actually reading the full stack trace points to _is_mailto being the culprit

julia/stdlib/Markdown/src/Common/inline.jl

Lines 151 to 156 in bb5b98e

	function _is_mailto(s::AbstractString)
	length(s) < 6 && return false
	# slicing strings is a bit risky, but this equality check is safe
	lowercase(s[1:6]) == "mailto:" || return false
	return occursin(_email_regex, s[6:end])
	end

ironically there's a comment specifying that it's a bit risky to be slicing the string (also it's a bit odd to check the first 6 characters with a 7 character string)

A suggestion would be to replace

julia/stdlib/Markdown/src/Common/inline.jl

Lines 154 to 155 in bb5b98e

	lowercase(s[1:6]) == "mailto:" || return false
	return occursin(_email_regex, s[6:end])

using first and nextind, I opened a PR with this here: #42140