vim:syn:md: Use lookarounds instead of \zs & \ze

Apparently the `\zs` token does not match characters if they are already
part of a syntax match. For example in `[^0][^1]` only the zero footnote
is matched. Fix this by using a positive lookbehind that does not have
this limitation.

Additionally, use a negative lookahead for the colon to make the pattern
slightly more readable, since it does not need the end-of-line check.

.config/vim/after/syntax/markdown.vim

@@ -0,0 +1,11 @@
+" Either not at the start of the line, or otherwise not followed by a colon
+syntax match markdownFootnoteZero  /\v(.@1<=\[\^0\])|(^\[\^0\]:@!)/ conceal cchar=⁰
+syntax match markdownFootnoteOne   /\v(.@1<=\[\^1\])|(^\[\^1\]:@!)/ conceal cchar=¹
+syntax match markdownFootnoteTwo   /\v(.@1<=\[\^2\])|(^\[\^2\]:@!)/ conceal cchar=²
+syntax match markdownFootnoteThree /\v(.@1<=\[\^3\])|(^\[\^3\]:@!)/ conceal cchar=³
+syntax match markdownFootnoteFour  /\v(.@1<=\[\^4\])|(^\[\^4\]:@!)/ conceal cchar=⁴
+syntax match markdownFootnoteFive  /\v(.@1<=\[\^5\])|(^\[\^5\]:@!)/ conceal cchar=⁵
+syntax match markdownFootnoteSix   /\v(.@1<=\[\^6\])|(^\[\^6\]:@!)/ conceal cchar=⁶
+syntax match markdownFootnoteSeven /\v(.@1<=\[\^7\])|(^\[\^7\]:@!)/ conceal cchar=⁷
+syntax match markdownFootnoteEight /\v(.@1<=\[\^8\])|(^\[\^8\]:@!)/ conceal cchar=⁸
+syntax match markdownFootnoteNine  /\v(.@1<=\[\^9\])|(^\[\^9\]:@!)/ conceal cchar=⁹