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

• [x] Bug fix (non-breaking change which fixes an issue)
• [x] 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

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

[codecov[bot]]

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

```diff @@ 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](https://app.codecov.io/gh/bubkoo/html-to-image/pull/407?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=%E5%B4%96) | Coverage Δ | | |---|---|---| | [src/embed-webfonts.ts](https://app.codecov.io/gh/bubkoo/html-to-image/pull/407?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=%E5%B4%96#diff-c3JjL2VtYmVkLXdlYmZvbnRzLnRz) | `30.63% <80.00%> (ø)` | |

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