feat: add mcpp build support (mcpp.toml)

[Sunrisepeak]

Summary

Adds an mcpp.toml manifest alongside the existing xmake.lua so
tinyhttps can be consumed/built via both ecosystems.

What lands

• mcpp.toml: declares modular C++23 lib tinyhttps exporting module
  mcpplibs.tinyhttps, sources src/**/*.cppm, runtime dep
  mbedtls 3.6.1 and dev dep gtest 1.15.2.

What's required upstream

A parallel PR adds the mbedtls 3.6.1 descriptor to
mcpp-community/mcpp-index.
That descriptor uses mcpp 0.0.2's C-language compile rule
(c_object) to compile all 108 of mbedtls's library/*.c into
libmbedtls.a, which mcpp's path-dep machinery then merges into
libtinyhttps.a. The two PRs land independently.

Verification

$ mcpp build
   Resolving toolchain
   ...
   Compiling mcpplibs.tinyhttps v0.2.0 (.)
      Cached mbedtls v3.6.1
    Finished release [optimized] in 1.52s

$ find target -name 'libtinyhttps.a'
target/x86_64-linux-musl/.../bin/libtinyhttps.a

$ ar t .../libtinyhttps.a | head
tls.m.o
sse.m.o
socket.m.o
proxy.m.o
http.m.o
ca_bundle.m.o
tinyhttps.m.o
x509write_csr.o
...

The archive contains all 6 module-interface objects plus mbedtls's
crypto + x509 + ssl translation units.

There's a benign warning from mcpp's module scanner about partition
imports (module 'mcpplibs.tinyhttps:http:tls' imported but not provided)
— it concatenates the source filename with the partition name when
guessing the logical name. GCC's dyndep step resolves it correctly,
the build/link succeed. Filed as a follow-up against mcpp-community/mcpp.