feat: add new @changesets/write package to separate writing changesets

[acheronfail]

This PR adds a new package @changesets/write which does the inverse of @changesets/parse.

It basically just extracts the writeChangesets function used in @changesets/cli so it can be re-used.

[changeset-bot]

🦋 Changeset is good to go

Latest commit: dcb09e6

We got this.

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[codecov]

Codecov Report

Merging #292 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff          @@
##           master   #292   +/-   ##
=====================================
  Coverage      80%    80%           
=====================================
  Files          45     45           
  Lines        1220   1220           
  Branches      280    284    +4     
=====================================
  Hits          976    976           
  Misses        233    233           
  Partials       11     11

Impacted Files	Coverage Δ
packages/cli/src/commands/add/index.ts	79.31% <ø> (ø)	⬆️
packages/write/src/index.ts	100% <ø> (ø)

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[Noviny]

Hey! Thanks for this.

We initially didn't do this because writing a changeset is simple enough that reimplementing it isn't really a cost, and having it as a library implies it's doing something fancier than it is.

What's the reason for wanting this as a package as opposed to just using the code yourself?

[Noviny]

PR otherwise looks fine, so happy to merge + release

[acheronfail]

We initially didn't do this because writing a changeset is simple enough that reimplementing it isn't really a cost, and having it as a library implies it's doing something fancier than it is.

Makes sense. I guess since we already have the @changesets/parse we've abstracted reading the changesets, why not abstract writing them too?

What's the reason for wanting this as a package as opposed to just using the code yourself?

We need a way to programmatically create a changeset (our use case is to integrate the renovate bot into our monorepo, and there needs to be a way to generate a changeset for the diff that the renovate bot creates).

It seemed odd that we had to duplicate the same logic that @changesets/cli itself was doing. Plus we wanted to have the nice things that it already does automatically, like human-id, integration with prettier, etc.