[Suggestion]: Allow `kind: group` entries to take `section_order`

Explain the feature

Allow using section_order on groups, and apply it to the listed files of the group.

Advantage

• Avoids repetition

Disadvantage

• More code to write
• Not that common of an issue
• Not clear what to do when both a group and a file from that group have an explicit and different section_order. Probably override.

Example(s)

Currently if we want to change the section order of all the files in a group it is needed to be set on every file, like this.

      - kind: group
        files:
          - { path: src/shapes.o, section_order: { .data: .rodata } }
          - { path: src/rcp_init.o, section_order: { .data: .rodata } }
          - { path: src/samples.o, section_order: { .data: .rodata } }

Ideally we could just tag the group itself and avoid repetition.

      - kind: group
        section_order: { .data: .rodata }
        files:
          - { path: src/shapes.o }
          - { path: src/rcp_init.o }
          - { path: src/samples.o }

Start by locating the configuration handling for kind: group entries and per-file section_order, using the YAML examples as the expected input shape. Define and implement how a group's mapping applies to its files, including the unresolved precedence when a file specifies a different mapping; done means grouped files no longer repeat the same setting and the conflict behavior is covered.