First implementation (#1)

Reviewed-on: #1

vectors/offsets.jsonl

@@ -0,0 +1,2 @@
+{"code_points":24,"exact":"$_2$O.","id":"bmp","plain":"A sentence about H$_2$O.","runs":[{"kind":"text","text":"A sentence about H"},{"kind":"math","notation":"tex","source":"_2"},{"kind":"text","text":"O."}],"span":[18,24],"utf16_span":[18,24],"utf16_units":24}
+{"code_points":22,"exact":"$𝑥$","id":"astral","plain":"Let $𝑥$ vary with $𝑦$.","runs":[{"kind":"text","text":"Let "},{"kind":"math","notation":"tex","source":"𝑥"},{"kind":"text","text":" vary with "},{"kind":"math","notation":"tex","source":"𝑦"},{"kind":"text","text":"."}],"span":[4,7],"utf16_span":[4,8],"utf16_units":24}