Support SPDX-FileName

[toobuntu]

reuse-tool already supports SPDX-FileCopyrightText and SPDX-FileContributor as defined in https://spdx.github.io/spdx-spec/v2.3/file-tags/.

We would like to also include SPDX-FileType and SPDX-FileName. The usefulness of these tags is still being debated, and we might drop them altogether. It can be argued that SPDX-FileName is largely irrelevant, but we would like to use it to canonicalize the full path and file name relative to the project root. To do this, it seems that we can add both to a Jinja template.

1. For SPDX-FileName, we would need a way to dynamically determine the file name for each file being processed. A variable to represent the current file could help.
2. For SPDX-FileType, we would seemingly need to use a conditional based on the path. E.g. {% if "/bin" in path %}SPDX-FileType: BINARY{% endif %}

Is this possible or out-of-scope?

For now we are including the information in REUSE.toml, which is silently ignored:

SPDX-FileType = "BINARY"
SPDX-FileName = "./bin/<executable-name>"