Relicense under `Apache-2.0 OR MIT`.

[xStrom]

Resvg is being relicensed under the Apache-2.0 OR MIT dual license. For for more background on that see #838.

The practical steps to make that happen are as follows:

• Readme license section updates for the repository root and for all published packages.
• LICENSE-APACHE and LICENSE-MIT files in the repository root and in the root of all published packages.
• LICENSE-SUMMARY.txt file for the explorer-thumbnailer installer. The txt extension may be required.
• Cargo.toml license info sourced from a single virtual workspace Cargo.toml license field.
• Authors now tracked in the AUTHORS file and referenced as the Resvg Authors.
• A new CI check to verify formatting of Rust source files, and existence of copyright headers in Rust/C/C++ source files.
• New standard Linebender copyright headers in all Rust/C/C++ source files, including auto-generated ones.
• More packages properly defined as workspace members in the virtual workspace Cargo.toml.
• A few formatting changes done by cargo fmt.
• Unix style line endings in explorer-thumbnailer.
• Changelog entry to notify users of the license change.

[xStrom]

Can't have explorer-thumbnailer be part of the workspace immediately, as it contains Windows-only code. Will have to deal with it later, as the solution is too large to be part of this PR.

[sorairolake]

In conjunction with this change, I suggest making to make the project compatible with the REUSE Specification. REUSE was started by the Free Software Foundation Europe (FSFE) to provide a set of recommendations to make licensing your Free Software projects easy, comprehensive, unambiguous, and machine-readable.

reuse can be used to help achieve and confirm this suggestion.