Implement isort's `default-section` setting (#10149)
## Summary
This fixes https://github.com/astral-sh/ruff/issues/7868.
Support isort's `default-section` feature which allows any imports that
match sections that are not in `section-order` to be mapped to a
specifically named section.
https://pycqa.github.io/isort/docs/configuration/options.html#default-section
This has a few implications:
- It is no longer required that all known sections are defined in
`section-order`.
- This is technically a bw-incompat change because currently if folks
define custom groups, and do not define a `section-order`, the code used
to add all known sections to `section-order` while emitting warnings.
**However, when this happened, users would be seeing warnings so I do
not think it should count as a bw-incompat change.**
## Test Plan
- Added a new test.
- Did not break any existing tests.
Finally, I ran the following config against Pyramid's complex codebase
that was previously using isort and this change worked there.
### pyramid's previous isort config
https://github.com/Pylons/pyramid/blob/5f7e286b0629b0a5f1225fe51172cba77eb8fda1/pyproject.toml#L22-L37
```toml
[tool.isort]
profile = "black"
multi_line_output = 3
src_paths = ["src", "tests"]
skip_glob = ["docs/*"]
include_trailing_comma = true
force_grid_wrap = false
combine_as_imports = true
line_length = 79
force_sort_within_sections = true
no_lines_before = "THIRDPARTY"
sections = "FUTURE,THIRDPARTY,FIRSTPARTY,LOCALFOLDER"
default_section = "THIRDPARTY"
known_first_party = "pyramid"
```
### tested with ruff isort config
```toml
[tool.ruff.lint.isort]
case-sensitive = true
combine-as-imports = true
force-sort-within-sections = true
section-order = [
"future",
"third-party",
"first-party",
"local-folder",
]
default-section = "third-party"
known-first-party = [
"pyramid",
]
```
