Version scheme

[max-privatevoid]

We should have an independent version scheme. I think a good approach would be a classic 3-number semver-style version starting at 1.0.0, where:

• The first number increments whenever a new upstream version is merged.
• The second number increments when downstream features are implemented.
• The third number increments when downstream bugfixes are implemented.

Technical details:

• The version shall be stored in crates/zedless/Cargo.toml.
• Build tools will extract the version from there, with the same mechanism as before.
• The editor should mention the upstream version in the "About Zedless..." dialog box (e.g. Based on Zed 0.198.0).