Add support for "tempel-trigger-prefix"

[rennsax]

The variable tempel-trigger-prefix controls which prefix need to be input first before the completion can recognize it. For example, if there is a snippet "header" returned by tempel--templates, with tempel-trigger-prefix set to "t", "theader" will then expand to the snippet body.

This variable should be respected by ACM too.

The effects:

With prefix set, no tempel completion options show up:

If the prefix is entered, tempel snippets can be matched:

TAB, the completion expands:

[manateelazycat]

thanks