Support CSS attributes

[amolenaar]

PR Type

What kind of change does this PR introduce?

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Documentation content changes

Issue Number: #284

What is the new behavior?

Define rules based on attributes.

Does this PR introduce a breaking change?

• Yes
• No

Other information

[sourcery-ai]

Sourcery Code Quality Report (beta)

❌ Merging this PR will decrease code quality in the affected files by 0.15 out of 10.

	Before	After	Change
Quality	9.3	9.15	-0.15
Complexity	0.9	1.18	0.28
Method Length	24.84	30.05	5.21
Lines	229	322	93

Changed files	Quality Before	Quality After	Quality Change
gaphor/core/modeling/diagram.py	9.3	9.15	-0.15 ⬇️
gaphor/core/modeling/tests/conftest.py	N/A	9.88	N/A
gaphor/core/modeling/tests/test_style_attributes.py	N/A	9.05	N/A

Please see our documentation here for details on how these metrics are calculated.

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