Support #[serde(transparent)] structs by generating type aliases

[xiao115255]

What & Why

A struct annotated with #[serde(transparent)] serializes as the value of its single non-skipped field, but typeshare still generated a wrapper struct for it. The generated definitions therefore did not match the actual serialized shape (per the examples in #26):

#[typeshare]
#[serde(transparent)]
pub struct Players {
    players: Vec<Player>,
}

Before:

export interface Players {
    players: Player[];
}

After:

export type Players = Player[];

Changes:

• core/src/parser.rs: when parsing a named struct with #[serde(transparent)] and exactly one non-skipped field, emit a RustItem::Alias to that field's type instead of a wrapper struct. This reuses the existing alias path already taken by single-field tuple structs, so all language backends get the correct behavior. Structs whose transparent field is overridden via #[typeshare(serialized_as)] are handled as well; structs that do not have exactly one non-skipped field fall back to the previous behavior (serde itself rejects those at compile time).
• core/data/tests/can_handle_serde_transparent/: new snapshot test covering both a single-field transparent struct and a transparent struct with an additional #[serde(skip)] field, across kotlin, swift, typescript, scala, go, and python.
• docs/src/usage/annotations.md: document the supported annotation.

Fixes #26

How Tested

• TDD: added the snapshot test first and confirmed it failed against the pre-change parser, then implemented the fix.
• cargo test --workspace: all 315 snapshot tests plus unit/doc tests pass (full suite, not just the new tests) — no regressions.
• cargo fmt --all -- --check: clean.
• cargo clippy --workspace --all-targets: no new warnings (37 pre-existing before and after).
• End-to-end: ran the CLI on the issue's exact scenario (cargo run -p typeshare-cli -- --lang=typescript --output-file=out.ts sample.rs) and confirmed it now emits export type Players = Player[];.

AI Disclosure

This PR was prepared with AI assistance (Claude Code), reviewed and submitted by @xiao115255.