feat(boil): Add floating-tag tool

[Techassi]

This PR adds a new command: boil tools floating-tag which takes and SemVer compatible input and turns it into a floating tag based on boil's rules. The main consumer will be stackabletech/actions.

$ boil tools floating-tag 1.2.3-rc.1
1.2-rc.1

$ boil tools floating-tag 1.2.3
1.2

$ boil tools floating-tag 0.0.0-dev
0.0.0-dev

This command only serves as a stepping stone to a more robust solution using structured data in boil-target-tags.json.

Initial sketch

{
  "krb": {
    // Always there
    "canonical": {
      "manifest": "",
      "index-manifest": ""
    },
    // Optional, only there with --floating-tag
    "floating": {
      "manifest": "",
      "index-manifest": ""
    },
    // Others, currently empty then. Probably will support something like --tag
    "others": [
      {
        "manifest": "",
        "index-manifest": ""
      }
    ]
  }
}

[NickLarsenNZ]

LGTM