feat: adds new configurable conventionalcommits preset

[bcoe]

Int his PR we introduce the conventionalcommits preset, here's why this is cool:

• the conventionalcommits preset is based directly on feat: release version 1.0.0-beta.4 of spec conventional-commits/conventionalcommits.org#141, allowing us to better handle the slight deviations from angular's convention, e.g., the recent introduction of a ! character for breaking changes.
• the conventionalcommits preset is highly configurable based on the specification described here.
  • this will allow standard-version to better support options other than GitHub in a well documented, standardized way.

BREAKING CHANGE: switches default preset to conventionalcommits

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This change is currently blocked by:

• releases currently blocked by conventional-changelog-preset-loader conventional-changelog#435, which is preventing us from deploying new versions of conventional-changelog.
• publish to npm conventional-changelog-config-spec#7, which would expose a list of configuration options for standard-version and other tools, so that it's easier to maintain consistent configuration options.

[bcoe]

@jbottigliero thanks for your amazing work defining https://github.com/conventional-changelog/conventional-changelog-config-spec, I'm really happy with how things turned out.

Try out standard-version@6.x.x, it's now using your spec.