add Rev2.4_en-US.md (Rev2.4 English translation)

[IepIweidieng]

I have translated the Rev2.4 into English.
However, previous revisions of Open Taiko Chart have not been translated, partially because of laziness but more because I want to confirm the terminologies and details before proceeding.

I did use DeepL translation (partially due to laziness) but have manually checked, fixed, and rephrased the texts. I am not fluent in the Japanese language, but I have checked the translation word-by-word.

See Open-Taiko-Chart-Rev2.4_en.md, in which I have marked all (if I did not forget to mark any) the additional words not existing in the Japanese version in the form of \[:word], which are added mainly to better fit English grammars while keeping roughly the same meaning.

Some translations which I feel might need to confirm:

• "gameplay" for "再生"/"playback"
• "format of ..." for "...の記法"; "notation of ..." for "...の書き方"
  • Not exact translations. I think that both might be able to translate to "notation", so I just chose different words to avoid conflicts.
• "難易度" is differentiated into "difficulty"/"difficulty name" and "difficulty star rating".
  • For the 説明/description for tcm.charts.difficulty, I am unsure whether the "1 番難易度が低いコース"/"the [:existing] difficulty with the lowest difficulty" means the difficulty with the lowest difficulty name or difficulty star rating. (I tentatively assumed it to be the difficulty name)
• "out-of-box" (means "available by default") for "そのまま"/"as-is"
  • I am unsure whether "そのまま" here means "the data can be deserialized without content modification" or "the data can be deserialized without defining custom methods or functions". (I tentatively assumed it to be the "without defining" meaning)
• "may NOT ..." or "NO ... may ..." for "...てはいけません"
  • Unsure about whether to use "should NOT" or "must NOT".
• "need to ..." for "...必要があります"
  • Unsure about whether to use "should" or "must".
  • In JSON, the path separator \ can be escaped as \\, but also as (the bad way) \u005c, so I felt that it might have been needed to avoid "must" here.
• Reduce the usage of quotes (" & ") but add for mentioning JSON field names in paragraphs.

※ Update d675c22: Fix grammar: "it notates ..." -> "... are notated in this file", for "このファイルには...記述されます"
※ Update 9d8943e: Fix plurality and prepositions;
　 remove redundant word "course" ("コース"/"course" in this sense has already been translated into "difficulty");
　 rephrase 説明/description for tcc.measures to prevent the verb;
　 fix typo "Infomation"
※ Update d62ad0d: Fix description of "songpreview", for "始まる時間" (relative to other (?)) vs. "開始する時間" (relative to self (?))