Add `format` parameter to `url_from_file_path` function and accept `/` in any Windows path

[rmisev]

The format parameter specifies the format of the given file path - POSIX or Windows. It can have any of the following values:

• upa::file_path_format::detect - detect file path format from first character: / - POSIX, otherwise - Windows
• upa::file_path_format::posix - POSIX file path format
• upa::file_path_format::windows - Windows file path format
• upa::file_path_format::native - the file path format corresponds to the OS on which the code was compiled

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (db24212) 99.72% compared to head (6aa27e7) 99.73%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #25 +/- ## ======================================= Coverage 99.72% 99.73% ======================================= Files 15 15 Lines 2217 2223 +6 ======================================= + Hits 2211 2217 +6 Misses 6 6 ``` | [Files](https://app.codecov.io/gh/rmisev/url_whatwg/pull/25?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Rimas+Misevi%C4%8Dius) | Coverage Δ | | |---|---|---| | [include/upa/url.h](https://app.codecov.io/gh/rmisev/url_whatwg/pull/25?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Rimas+Misevi%C4%8Dius#diff-aW5jbHVkZS91cGEvdXJsLmg=) | `99.74% <100.00%> (+<0.01%)` | :arrow_up: |

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