[FIX] parser: make t-on stricter

[ged-odoo]

Before this commit, the t-on directive assumed that the next character would be a -, and ignored it, so if someone would write t-onclick by mistake, Owl would then add an event handler on the lick event, instead of click.

With this commit, we improve the parser to make it stricter and fail if there is no dash.

closes #1441

[ged-odoo]

@sdegueldre

[ged-odoo]

@brboi