Add TodoMVC for dart2js & dart2wasm

[mkustermann]

This is a Jaspr Dart Web application compiled with dart2js and dart2wasm.

The source is from [0] and was compiled with a modified Jaspr CLI tool (see PR in [1]) using the following commands:

% rm -rf build
% dart run --resident <jaspr>/packages/jaspr_cli/bin/jaspr.dart build -O4 --extra-js-compiler-option=--no-minify

% rm -rf build
% dart run --resident <jaspr>/packages/jaspr_cli/bin/jaspr.dart build -O4 --extra-wasm-compiler-option=--no-strip-wasm --experimental-wasm

The relevant files in build/jaspr/* were copied to the Speedometer benchmark folders.

Used Dart SDK version is: 3.8.0-edge.4c8aedcb57fe678e6d30acfdc021aa9888576638

Before landing we should figure out:

* do we want both dart2js and dart2wasm?
* do we want -O2 or -O4?
* can we rely on `js-string` builtin to be evailable or not?

[0] https://github.com/mkustermann/todomvc [1] https://github.com/schultek/jaspr/pull/397