Support for natural language accents to `styling::to_style`

[MDLC01]

Precomposed accented characters such as "é" should first be decomposed, and only then styled.

My interpretation of UTR #25, Section 6.5 Accented Characters, and especially the sentence below, is that we should first decompose any precomposed character, and only then apply the styles.

to achieve consistent results, a mathematical display system should transiently decompose any precomposed upright letters when used in mathematical expressions, and should use a single algorithm to place embellishments.