feat: add custom Mermaid configuration support

[ayhansipahi]

Description

Implements the ability to pass custom properties during Mermaid initialization as requested in issue #120. Users can now customize themes, colors, fonts, and other Mermaid rendering options.

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Performance improvement
• Refactoring (no functional changes)

Related Issues

Closes #120

Changes Made

• Add mermaidConfig prop to Streamdown component - Use official MermaidConfig type from mermaid package - Pass config through React Context to Mermaid components - Update documentation with usage examples - Add comprehensive tests for configuration handling

Testing

• All existing tests pass
• Added new tests for the changes
• Manually tested the changes

Test Coverage

 Test Files  10 passed (10)
      Tests  217 passed (217)
   Start at  00:06:34
   Duration  2.49s (transform 550ms, setup 2.60s, collect 4.00s, tests 1.84s, environment 5.85s, prepare 953ms)

 % Coverage report from v8
-------------------------------|---------|----------|---------|---------|-----------------------------------------------
File                           | % Stmts | % Branch | % Funcs | % Lines | Uncovered Line #s                             
-------------------------------|---------|----------|---------|---------|-----------------------------------------------
All files                      |   81.61 |    93.31 |      80 |   81.61 |                                               
 streamdown                    |     100 |      100 |     100 |     100 |                                               
  index.tsx                    |     100 |      100 |     100 |     100 |                                               
 streamdown/lib                |   80.55 |    92.95 |      80 |   80.55 |                                               
  code-block.tsx               |   98.57 |    93.33 |      90 |   98.57 | 90,576-582                                    
  components.tsx               |      98 |    86.84 |     100 |      98 | 53-56,58                                      
  image.tsx                    |     4.1 |      100 |       0 |     4.1 | 13-98                                         
  mermaid.tsx                  |   98.91 |     90.9 |     100 |   98.91 | 81                                            
  parse-blocks.tsx             |     100 |       75 |     100 |     100 | 19,32-33                                      
  parse-incomplete-markdown.ts |   94.49 |    96.51 |   94.44 |   94.49 | 73-74,370-373,419-433                         
  table.tsx                    |   23.44 |       75 |   16.66 |   23.44 | 10-88,111-160,195-267,286-314,318-324,346-361 
  utils.ts                     |   28.57 |      100 |      50 |   28.57 | 7-16                                          
-------------------------------|---------|----------|---------|---------|-----------------------------------------------
└─ streamdown#test:coverage ──

 Tasks:    1 successful, 1 total
Cached:    0 cached, 1 total
  Time:    3.342s 

Screenshots/Demos

Checklist

• My code follows the project's code style
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings or errors
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• I have created a changeset (pnpm changeset)

Changeset

• I have created a changeset for these changes

Additional Notes

[haydenbleasel]

@ayhansipahi legendary improvement 🚀