Upstream: markdown-it-texmath 1.0.0 (MIT), texmath.js.

Local blockquote fix:
Use open blockquote tokens instead of state.parentType to detect quoted math.
After a prose paragraph, parentType may be "paragraph" even inside a quote.
Remove at most the active quote depth of leading > markers per line, preserving
newlines and any additional > that belongs to the formula. Parsing rules and
delimiter expressions are otherwise unchanged.

Block-rule registration:
Add markdown-it's alt contexts (paragraph/reference/blockquote/list) so block
math can interrupt prose without an empty line. This is registration metadata;
the upstream parsers are unchanged and each rule is registered only once.
