[Enhance] slant relation symbols: \leq, \geq, \nleq and \ngeq

[Firestar-Reimu]

Request to add an option of using slant relation symbols:

$\leq$ vs $\leqslant$ $\geq$ vs $\geqslant$ $\nleq$ vs $\nleqslant$ $\ngeq$ vs $\ngeqslant$