infinitives - What part of speech is the word "to" in "Alice likes to dance"?

Annie likes to dance

What part of speech is the word to?

Reopen note:

Merriam Webster lists it as a preposition and gives the following definition:

8 —used as a function word to indicate that the following verb is an infinitive < wants to go > and often used by itself at the end of a clause in place of an infinitive suggested by the preceding context < knows more than she seems to >

It doesn't seem to very convincing that to is a preposition when it occurs before a verb like this.