Enable writing stack trees to PR bodies

[oleg-codaio]

This PR adds a new configuration option, pullRequest.writeStack, that if enabled will add stack trees directly to GitHub pull requests.

For ease of navigation, these are added to the top as follows:

And when expanded, show the stack of requests from the latest PR down to the trunk branch:

Most PR stacks tend to be linear, but an alternate format is used if there are forks in the stack that make it look more like a tree:

This isn't the default format since I personally think the one above is cleaner to read, especially on phones that run out of horizontal real estate.

[aviator-app]

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[aviator-app]

FlexReview Summary

Based on the code complexity and the author's expertise score, these are the suggested reviewers:

• @draftcode (current review load: 4)

See the list of alternate reviewers in the detailed breakdown below.

Detailed Breakdown

Author’s expertise score for the modified files:

• cmd/av/pr_create.go (0.00)
• cmd/av/stack_submit.go (0.00)
• internal/actions/pr.go (0.00)
• internal/actions/pr_test.go (0.00)
• internal/actions/sync_branch.go (0.00)
• internal/config/config.go (0.00)
• internal/meta/branch.go (0.00)
• internal/utils/stackutils/stackutils.go (0.00)

Files	Reviewers
cmd/av/pr_create.go †, internal/actions/sync_branch.go †, internal/config/config.go †	@draftcode (score: 1.00, current review load: 4), @jainankit (score: 0.45, current review load: 0), @doratzeng (score: 0.41, current review load: 0)

† Indicates that the file doesn't need an expert review. (?)

See full breakdown of the reviewers on the Aviator webapp.

[draftcode]

I'll cut a new release for av-cli.