Stabilise exports field support

[emmatown]

It seems to be working quite well on various projects so it's probably good enough to be stable.

Closes #251
Closes #431
Closes #432

https://preconstruct.tools/configuration#exports

[changeset-bot]

🦋 Changeset detected

Latest commit: a29a1f4

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@preconstruct/cli	Minor

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[codecov]

Codecov Report

Base: 91.19% // Head: 91.18% // Decreases project coverage by -0.01% ⚠️

Coverage data is based on head (a29a1f4) compared to base (c32a4de).
Patch has no changes to coverable lines.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #508      +/-   ##
==========================================
- Coverage   91.19%   91.18%   -0.02%     
==========================================
  Files          33       33              
  Lines        1477     1475       -2     
  Branches      396      404       +8     
==========================================
- Hits         1347     1345       -2     
  Misses        124      124              
  Partials        6        6              

Impacted Files	Coverage Δ
packages/cli/src/package.ts	95.20% <ø> (-0.07%)	⬇️
packages/cli/src/project.ts	79.62% <ø> (ø)
packages/cli/src/validate.ts	91.46% <ø> (ø)

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