[MinePay] Top Donator placeholder support added.

MinePay now registers placeholders with PlaceholderAPI (if installed),
allowing you to display live top donator data in holograms, scoreboards,
tab lists, and any other PAPI-compatible plugin.

PlaceholderAPI is optional, MinePay works normally without it.

━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━
LEADERBOARD PLACEHOLDERS
━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━
```
-- Replace <n> with the position (1, 2, 3, etc.)

Monthly:
%minepay_monthly_name_<n>% Top donor name
%minepay_monthly_amount_<n>% Total donated (e.g. 25.00)
%minepay_monthly_count_<n>% Number of purchases
%minepay_monthly_lastdonation_<n>% Last donation date
%minepay_monthly_period% Current period (e.g. May 2026)

All-Time:
%minepay_alltime_name_<n>%
%minepay_alltime_amount_<n>%
%minepay_alltime_count_<n>%
%minepay_alltime_lastdonation_<n>%
```

━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━
PLAYER SELF PLACEHOLDERS
━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━
```
-- These reflect the stats of whichever player is viewing the text.

%minepay_monthly_rank% Your monthly rank (or "Unranked")
%minepay_monthly_donated% Your monthly total donated
%minepay_monthly_count% Your monthly purchase count

%minepay_alltime_rank%
%minepay_alltime_donated%
%minepay_alltime_count%
```

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EXAMPLE HOLOGRAM LINES
━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━
```
&6&l#1 &f%minepay_monthly_name_1% &8- &a$%minepay_monthly_amount_1%
&e&l#2 &f%minepay_monthly_name_2% &8- &a$%minepay_monthly_amount_2%
&c&l#3 &f%minepay_monthly_name_3% &8- &a$%minepay_monthly_amount_3%
```

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NOTES
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• Data is cached and refreshed every 30 minutes automatically.
• If a slot is empty (e.g. fewer than 10 donors this month),
the placeholder returns blank rather than an error.
• Requires PlaceholderAPI to be installed on the server.
MinePay functions normally if it is not present.