Make releasing process a GitHub workflow

[mrkn]

I used to release new versions of the gem by running the rake release command in my local terminal.

Since #250, we have supported JRuby through a gem targeting the Java platform. However, I always forgot that it needs to be released separately from the CRuby version.

I reflected on why I keep forgetting and identified two reasons:

1. I've been releasing many versions for a long time using a single rake release command.
2. I rarely use JRuby and don't have a suitable environment for it.

Defining a workflow to release both versions together would help prevent this issue.

[hsbt]

I can help this same as https://github.com/ruby/psych/blob/master/.github/workflows/push_gem.yml

After that, we can publish new releases of ruby and java platforms with git tag and pushing that tags.

[mrkn]

@hsbt Sounds great! Could you open a pull request?

[hsbt]

I created the trusted publisher for bigdecimal now.