feat: Support Unicode (sub|super)script characters (#3633)

* feat: Support Unicode (sub|super)script characters

* Acquire tokens via repeated fetch()

* Match more Unicode (sub|super)script characters

* Update docs with new characters

* Add Greek characters to RegEx

* Pick up review comments

Co-authored-by: Erik Demaine <edemaine@mit.edu>

docs/supported.md

@@ -190,6 +190,9 @@ $\allowbreak α β γ δ ϵ ζ η θ ι κ λ μ ν ξ o π \allowbreak ρ σ τ
 
 Direct Input: $∂ ∇ ℑ Ⅎ ℵ ℶ ℷ ℸ ⅁ ℏ ð − ∗$
 ÀÁÂÃÄÅÆÇÈÉÊËÌÍÎÏÐÑÒÓÔÕÖÙÚÛÜÝÞßàáâãäåçèéêëìíîïðñòóôöùúûüýþÿ
+₊₋₌₍₎₀₁₂₃₄₅₆₇₈₉ₐₑₕᵢⱼₖₗₘₙₒₚᵣₛₜᵤᵥₓᵦᵧᵨᵩᵪ⁺⁻⁼⁽⁾⁰¹²³⁴⁵⁶⁷⁸⁹ᵃᵇᶜᵈᵉᵍʰⁱʲᵏˡᵐⁿᵒᵖʳˢᵗᵘʷˣʸᶻᵛᵝᵞᵟᵠᵡ
+
+Math-mode Unicode (sub|super)script characters will render as if you had written regular characters in a subscript or superscript. For instance, `A²⁺³` will render the same as `A^{2+3}`.
 
 </div>
 <div class="katex-cards" id="math-alpha">