fix: parse WebVTT cues with settings

[itboy87]

Summary

• allow WebVTT cue timing lines to contain standard cue settings
• preserve existing parsing behavior for timing and subtitle text
• add regression coverage for STYLE metadata and line/vertical settings

Previously, the parser required the first timing line to fully match a timestamp-only regex. Valid lines such as:

00:00:01.000 --> 00:00:03.000 line:0

were skipped, resulting in an empty cue list.

Testing

• ./gradlew :mediaplayer:jvmTest --tests 'io.github.kdroidfilter.composemediaplayer.subtitle.WebVttParserTest'
• ./gradlew :mediaplayer:ktlintCheck

Reproduction samples

These public WebVTT files reproduce the issue:

• Positioned numeric cues
• Japanese vertical cues

They contain valid cue settings such as line:0, line:-1, and vertical:rl. Previously, the parser skipped the first timing line because it required the timestamp-only pattern to match the entire line.