This modifies the Resource class to use Optional<String> for the
encoding. If the encoding is unknown, the Optional will not have a
value (instead of using the null state of the String class). It also
implements a has_encoding() instance method and modifies the callers
of Resource::encoding() to use the new API.
SPDX License Identifiers are a more compact / standardized
way of representing file license information.
See: https://spdx.dev/resources/use/#identifiers
This was done with the `ambr` search and replace tool.
ambr --no-parent-ignore --key-from-file --rep-from-file key.txt rep.txt *
