query: allow custom delimiter for nested structs

[AdamMagued]

Summary

Currently, nested struct fields are scoped strictly using bracket notation (user[name]=acme&user[addr][postcode]=1234). While slices support custom delimiters via the del tag and named options (comma, space), nested struct scoping did not allow configuring a custom separator such as dot notation (user.name=acme&user.addr.postcode=1234).

This change adds support for configuring the delimiter used for scoping nested struct fields:

• Specifying the del tag (e.g. del:".", del:"/") on a nested struct field uses that delimiter for scoping child fields.
• Specifying the dot option in the url tag (e.g. url:"user,dot") sets the delimiter to ..
• If a parent struct field defines a delimiter, nested sub-structs inherit that delimiter unless explicitly overridden.
• When no delimiter is specified, the default bracket notation (scope[name]) remains unchanged.

Root Cause Analysis

In query/encode.go, reflectValue hardcoded bracket-based scoping when scope != "":

if scope != "" {
    name = scope + "[" + name + "]"
}

There was no mechanism to customize the separator for struct fields, unlike slices which inspect sf.Tag.Get("del") and tag options.

Verification Results

• All existing test cases pass without modification.
• Added unit tests covering:
  • Custom delimiter via del:"." struct tag.
  • Custom delimiter via url:",dot" option.
  • Custom delimiter with arbitrary separator (del:"/").
  • Multi-level nested structs matching the issue How to encode nested structure by my way? #79 example (user.name, user.addr.postcode, user.addr.city).
• Test suite passes with 100.0% statement coverage:
  go test -v -cover ./... -> coverage: 100.0% of statements.
• go vet ./... and gofmt run cleanly with zero warnings.

Fixes #79

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