Support for `season` and `season-0X` in `date-parts`

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Description

The CSL JSON schema supports expressing seasons using:

1. A dedicated season property (e.g. "season": 4)
2. A seasonal token in date-parts (e.g. [2009, "season-04"])

Reference:
https://github.com/citation-style-language/schema/blob/master/schemas/input/csl-data.json

However, citeproc-php currently does not render either form correctly. If "season-0X" is used in date-parts, it is treated as an invalid or unknown month, and if season is set separately, it is ignored entirely.

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Steps to Reproduce

Use the following CSL JSON input:

{
  "issued": {
    "date-parts": [[2009, "season-04"]],
    "season": 4
  }
}

With a CSL style such as:

<date variable="issued" form="text">
  <date-part name="month" suffix=" "/>
  <date-part name="year"/>
</date>

Expected output:

Winter 2009

Actual output:

Mar. 2009

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Expected Behavior

The processor should interpret "season-04" as a valid placeholder in date-parts (like citeproc-js), and/or render the season property if date-parts does not contain a second value.

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Environment

• citeproc-php version: "seboettg/citeproc-php": "2.6.2"
• PHP version: PHP 8.3.14

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Proposed Fix

• Add support for parsing "season-0X" tokens in date-parts, with localization via <term name="season-04">Winter</term> and related terms.
• Optionally support the season property as a fallback or primary field when only the year is given.

This would bring citeproc-php into alignment with the CSL JSON specification and improve compatibility with existing citation processors like citeproc-js.

Links:

• https://docs.citationstyles.org/en/stable/specification.html#seasons
• https://github.com/citation-style-language/locales/blob/606fa26be1d87837c4e607362b28ed58a7576875/locales-en-US.xml