Squeak bootstrap

[LinqLover]

Relates #524.

/cc @tom95

Next steps:

• [x] Update SAR file on https://files.squeak.org/metacello/metacello-squeak60.sar
• [x] Update Installer class >> #ensureRecentMetacello as described in https://github.com/Metacello/metacello/issues/524#issue-672970494
• [x] Discuss whether Metacello-Help should be part of the bootstrap archive

In a second step:

• [ ] Add CD script to deploy the latest SAR files for Squeak 6.0 and newer to GitHub Releases/squeak.org
• [ ] Reduce Installer class >> #ensureRecentMetacello so that no further update is required after installing the SAR file

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For convenience, here are the generation outputs:

• metacello-squeak60.zip

[LinqLover]

@dalehenrich How would you feel about merging this PR as-is? It only adds a SAR generation script for now, and we can address a complete CD script in a later step (see list above). As that second step currently seems not to have priority at the moment, I fear this branch might get lost otherwise.