[j] and [w] are considered semivowels a.k.a. glides, in French either semi-voyelles or semi-consonnes.

In poetry, syllables would be split that way:

• lu•mière or lu•miè•re
• pre•mier
• boire or boi•re
• jou•et

Lumière and boire have one less syllable when followed by a vowel, a mute h or at the end of a verse.

Note that in poetry jouet changes its pronunciation from the monosyllabic [ʒwɛ] to to the disyllabic [ʒu.ɛ]