feat: read dedicated config files from their top level

[bhirsz]

Adds top_level_config_names: config files dedicated to the tool that may skip the [tool.<name>] table.

class RobotArgs(ArgConfig):
    config_names = ["robot.toml", "pyproject.toml"]
    top_level_config_names = ["robot.toml"]

# robot.toml (robotcode-compatible layout)
outputdir = "results"
[variable]
BROWSER = "chrome"
[profiles.ci]
loglevel = "DEBUG"

• The [tool.<name>] section still wins when present ("accept both"). A file with both the section and other top-level keys raises an error.
• The top-level tool table (other tools' settings, e.g. [tool.robotcode-analyze]) is ignored.
• A dedicated file with nothing for us (only [tool.other]) is skipped, and discovery continues.
• Only listed names are top-level during discovery. So rebot with top_level_config_names = ["rebot.toml"] still reads only [tool.rebot] from a shared robot.toml.
• --config FILE and extends targets may use the top level too, unless the file is one of the non-dedicated config_names (e.g. pyproject.toml).
• Discovery is unchanged otherwise: the first project file that defines the config wins, and files are never merged.

Needed by robotframework/robotframework#5773 so robot.toml can use robotcode's top-level layout. Stacked on #50.