feat: support extra content which will be added to style tag from svg

[2knu]

Description

Add a new parameter extraStyleContent which can be used to pass extra css code to resulting svg.

Motivation and Context

This change can be used to fix the styling part of #345

Types of changes

• 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 change)
• Enhancement (changes that improvement of current feature or performance)
• Refactoring (changes that neither fixes a bug nor adds a feature)
• Test Case (changes that add missing tests or correct existing tests)
• Code style optimization (changes that do not affect the meaning of the code)
• Docs (changes that only update documentation)
• Chore (changes that don't modify src or test files)

Self Check before Merge

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

[codecov]

Codecov Report

Patch coverage: 80.00% and no project coverage change.

Comparison is base (b751cbf) 62.93% compared to head (772ca32) 62.93%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #407   +/-   ##
=======================================
  Coverage   62.93%   62.93%           
=======================================
  Files          10       10           
  Lines         580      580           
  Branches      143      143           
=======================================
  Hits          365      365           
  Misses        151      151           
  Partials       64       64           

Impacted Files	Coverage Δ
src/embed-webfonts.ts	30.63% <80.00%> (ø)

☔ View full report in Codecov by Sentry.
📢 Do you have feedback about the report comment? Let us know in this issue.