Allow setting a tag prefix manually

[sloede]

This PR enables users to manually specify a tag prefix, overriding the default tag prefix determination algorithm (the DTPDA, as it will henceforth be referred to as).

Before:

• subdir="": tag prefix is v, e.g., v0.1.0
• subdir="foo": tag prefix is foo-v, e.g., foo-v0.1.0

With this PR, there are additional options available through the tag_prefix input:

• tag_prefix="" (default): as before
• tag_prefix="bar": tag prefix is bar-v, e.g., bar-v0.1.0 (whatever the content of subdir)
• tag_prefix="NO_PREFIX": tag prefix is v, e.g., v0.1.0 (whatever the content of subdir)

The main motivation is to allow users who have their Julia package in a subdirectory but want to tag their versions just as v0.1.0 and not as subdir-v0.1.0 to use TagBot (which is an awesome action, thanks a lot to all maintainers!). This should be a non-breaking change, since the DTPDA remains unchanged if the new action input tag_prefix is not set.

Resolves #278.

cc @bgeihe

[sloede]

bump Who would be the right person to do a (first) review for this PR - @christopher-dG?

[christopher-dG]

Looks good at a glance, but would be good to get review from @IanButterworth

[sloede]

Thanks for the merge @IanButterworth! It would be great if you could also create a new tag (I don't think I can do this as a PR, or can I?), such that it is usable in our workflows.

[IanButterworth]

Yeah. I've messaged @christopher-dG because there's more to it than a PR AFAIK but I don't know what that is

[sloede]

OK, great, thanks a lot!