[Fix] Fix rendering for non-braced LaTeX subscripts and superscipts

[nil0x9]

Non-braced subscripts and superscripts such as _t and ^2 were not rendered
because their lookup conditions used item.text[1]:sub(3).

For inputs like _t or ^2, sub(3) returns an empty string, so the condition
fails even when the target character exists in symbols.subscripts or
symbols.superscripts. The actual render paths already use
item.text[1]:sub(2), which correctly extracts the character after _ or ^.

This change makes the lookup conditions use the same substring offset as the
render lookup, allowing expressions like M_t, G_t, and x^2 to render
consistently with braced forms like M_{t} and x^{2}.

LaTex command	Before	After
M_t = \beta M_{t-1} + (1-\beta)G_t
y^2 + y^2 \leq 1

[OXY2DEV]

Thanks for the PR!