Add money filters

[lahma]

Fixes #238

Implements the Shopify money filters, registered with the opt-in FilterCollection.WithMoneyFilters() (like WithColorFilters(), they are not added by the TemplateOptions constructor):

Filter	Source	Result
money	{{ 1134.65 | money }}	$1,134.65
money_with_currency	{{ 1134.65 | money_with_currency }}	$1,134.65 USD
money_without_currency	{{ 1134.65 | money_without_currency }}	1,134.65
money_without_trailing_zeros	{{ 10.00 | money_without_trailing_zeros }}	$10

Configuration

A new TemplateOptions.MoneyOptions, mirrored on TemplateContext so a currency can be selected per rendering:

• Culture based by default. The currency and the separators come from TemplateOptions.CultureInfo. The invariant culture has no region, so USD/$ is used, which matches Shopify's default ${{amount}} format.
• Currency, an ISO 4217 code, also accepted as a filter argument: {{ 10 | money: 'EUR' }} and {{ 10 | money: currency: 'EUR' }}. Symbols and decimal digits come from an extensible Currencies collection seeded with the common codes; an unregistered code is rendered using the code itself as the symbol.
• AmountsInCents (default false) divides the input by 100, for templates ported from Shopify where prices are stored as integers.
• MoneyFormat / MoneyWithCurrencyFormat override the culture based formatting with the Shopify currency formatting placeholders ({{amount}}, {{amount_with_comma_separator}}, …), plus {{currency}} and {{currency_symbol}} so a single format works with a currency resolved at rendering time. Formats are parsed once, when assigned.

Notes

Amounts default to plain decimals rather than cents. The issue comment on #238 is a good illustration of what people expect: {{ item.Price | money_without_currency }} currently falls through unfiltered and renders 31.300 or 31.30 depending on the scale of the decimal that came out of the database. Money formatting pins the decimal count, and there is a test for it.

Two deliberate deviations from raw .NET currency formatting, both overridable with a format string:

• Amounts are rounded away from zero, as Shopify does, instead of .NET's banker's rounding.
• Negative amounts render as -$10.00 rather than the accounting ($10.00) that "C" produces for en-US and the invariant culture.

README.md has a new "Money filters" section, and CLAUDE.md is added.

Testing

70 new tests in Fluid.Tests/MoneyFiltersTests.cs. Full suite passes on both the regular and the Compiled=true pass, and the build is clean on all four TFMs including netstandard2.0.

[hishamco]

Just a little note before reviewing why we simplify this by having one money filter, then use arguments to implement the others

[sebastienros]

AGENTS.md instead?

[lahma]

AGENTS.md instead?

Merged into AGENTS.md