annotations: Distribute the license with the crate

[dm0-]

Description

This just adds license-file to the annotations crate pointed at the license in the parent directory. Since rust-lang/cargo#7905, this will distribute the license text in the crate so it is available with the code fetched from crates.io. (It could be added to the other Cargo.tomls, too, but mirai-annotations is the only one I see being published on crates.io from a quick search.)

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• API change with a documentation update
• Additional test coverage
• Code cleanup or just keeping up with the latest Rustc nightly

How Has This Been Tested?

Run cargo package --list to verify the LICENSE file will be added, and also just cargo package to inspect it.

Checklist:

• Fork the repo and create your branch from main.
• If you've added code that should be tested, add tests.
• If you've changed APIs, update the documentation.
• Ensure the test suite passes.
• Make sure your code lints.
• If you haven't already, complete your CLA here: https://code.facebook.com/cla

[dm0-]

I've updated this to use a symlink to copy the license into the crate instead, since defining license-file semantically means it's a nonstandard license apparently. If symlinks should be avoided, the file could be copied.

[codecov]

Codecov Report

Merging #1221 (caa26d2) into main (41b3c94) will increase coverage by 0%.
The diff coverage is n/a.

@@          Coverage Diff          @@
##            main   #1221   +/-   ##
=====================================
  Coverage     74%     74%           
=====================================
  Files         23      23           
  Lines      16537   16537           
=====================================
+ Hits       12337   12338    +1     
+ Misses      4200    4199    -1     

see 1 file with indirect coverage changes

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