Implement follow-symlinks boolean action input

[p--b]

This commit implements a new boolean input to the action,
follow-symlinks. This option configures whether the glob expansion
will follow any symlinks it finds when determining the set of of files
to be archived into the artifact.

The default value of the option, which preserves the existing behaviour,
is true. When set to true, symbolic links will be be followed and expanded.
If false, symbolic links will be included in the archived artifact verbatim.

Users may wish to set this option to false if their artifact contains
internally-referencing symlinks which would result in significant bloat
(and semantic change!) in the source files when the artifact is created.

Resolves: #93.

[p--b]

Having just tried this out in a (long and slow) pipeline that generates lots of symlinks, upstream at https://github.com/actions/toolkit/ throws an error if passed a symlink to ArtifactClient.uploadArtifact(...);

Error: EISDIR: illegal operation on a directory, read
Emitted 'error' event on ReadStream instance at:
    at internal/fs/streams.js:167:12
    at FSReqCallback.wrapper [as oncomplete] (fs.js:470:5) {
  errno: -21,
  code: 'EISDIR',
  syscall: 'read'
}

In turn I believe this is because of this:

https://github.com/actions/toolkit/blob/cac7db2d19f7a7e48fe1ec608fc99bc1786a858e/packages/artifact/src/internal/upload-gzip.ts#L43

It would need to be passed fs.constants.O_RDONLY | fs.constants.O_SYMLINK to its flags options property in order to not traverse the symlink. If I get round to it I'll file an issue over there, after which this PR might become useful. Sorry for the noise.