Add mbstring support to the web php wasm

[akirk]

This is just a first attempt to see if its feasible. It adds about 800kb to the wasm.

[adamziel]

The PR is great, thank you @akirk!

At the same time, 800kb is quite a bit. Here are the numbers after compression:

 php_8_1.wasm: 6.5M vs 5.7M
 php_8_1.wasm.gz: 2.6M vs 2.0M
 php_8_1.wasm.br:  1.9M vs 1.5M

At this point I think a web-full build would make more sense and it could also include libxml that adds another 1M. Technically, it would require another preset in build.js and another recompile script in packages/php-wasm/web/project.json. Once that's in place, the getPHPLoaderModule() function could be updated to support versions like 8.2-full (or to accept tag=full).

[adamziel]

This got a bit stale so let's close it for now.