feat(template): add FastParser for TinyGo compatibility

[cxjava]

Summary

Add FastParser as an alternative template parser using valyala/fasttemplate, primarily for TinyGo compatibility.

Motivation

TinyGo has limited support for the standard library's text/template and html/template packages due to their heavy use of reflection and other features that are difficult to compile to WebAssembly or embedded targets. This makes go-i18n unusable in TinyGo environments when templates contain placeholders.

FastParser provides a lightweight alternative that works reliably in TinyGo while still supporting the common use case of simple placeholder substitution.

Changes

• Add FastParser implementing template.Parser interface
• Add comprehensive unit tests for FastParser
• Add integration tests in localizer_test.go
• Add github.com/valyala/fasttemplate dependency

Usage

localizer.Localize(&i18n.LocalizeConfig{
    DefaultMessage: &i18n.Message{
        ID:    "HelloPerson",
        One:   "{{.Name}} has {{.Count}} cat",
        Other: "{{.Name}} has {{.Count}} cats",
    },
    TemplateData: map[string]interface{}{
        "Name":  "Nick",
        "Count": 2,
    },
    PluralCount:    2,
    TemplateParser: &template.FastParser{},  // Use FastParser
})

Supported Features

• Simple placeholder substitution: {{.Key}} and {{Key}}
• Custom delimiters (e.g., << >>)
• Data types: map[string]interface{}, map[string]string, and structs
• All plural forms (Zero, One, Two, Few, Many, Other)
• Missing keys preserve the original placeholder

Limitations

For these features, continue using TextParser:

• Template functions (e.g., {{printf "%s" .Name}})
• Conditionals (e.g., {{if .Cond}}...{{end}})
• Range loops (e.g., {{range .Items}}...{{end}})
• Nested field access (e.g., {{.User.Name}})
• Method calls (e.g., {{.Name.String}})

Benchmark

BenchmarkFastParser_Execute-14    249.3 ns/op    368 B/op    10 allocs/op
BenchmarkTextParser_Execute-14    263.1 ns/op    336 B/op     9 allocs/op

FastParser is slightly faster (~5%) for simple placeholder substitution.

[codecov]

Codecov Report

❌ Patch coverage is 75.75758% with 24 lines in your changes missing coverage. Please review.
✅ Project coverage is 74.24%. Comparing base (4d19ec6) to head (50e3968).

Files with missing lines	Patch %	Lines
i18n/template/fast_parser.go	75.75%	17 Missing and 7 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #400      +/-   ##
==========================================
+ Coverage   73.97%   74.24%   +0.26%     
==========================================
  Files          20       21       +1     
  Lines        1679     1778      +99     
==========================================
+ Hits         1242     1320      +78     
- Misses        376      389      +13     
- Partials       61       69       +8     

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