Add support for "end of the week/month" expressions

[Copilot]

The parser recognized "end of this/next week" and "end of this/next month" but failed to parse "end of the week" and "end of the month" — where "the" refers to the current period.

Changes

• DateParser.g: Added THE WEEK alternative to explicit_relative_week_seek and THE MONTH to explicit_relative_month_seek, both resolving to seek offset 0 (current period), consistent with how THIS/CURRENT prefixes are handled.
• DateGrammarTest.java: Added AST assertions for end/beginning of the week/month.

Behavior

"end of the week"       → same as "end of this week"
"end of the month"      → same as "end of this month"
"beginning of the week" → same as "beginning of this week"
"beginning of the month"→ same as "beginning of this month"

Original prompt

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This section details on the original issue you should resolve

<issue_title>[Feature] Add support for "End of ..."</issue_title>
<issue_description>Allow to parse expressions like

"End of the week"

"End of the month"</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes [Feature] Add support for "End of ..." #133

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