Adds `while` and most of the expression support. Splits apart the fixity test to demonstrate more closely which bits are still failing.
```
//toolchain/parser/testdata:basics/fail_paren_match_regression.carbon.test FAILED in 0.8s
//toolchain/parser/testdata:basics/function_call.carbon.test FAILED in 0.7s
//toolchain/parser/testdata:basics/package.carbon.test FAILED in 0.7s
//toolchain/parser/testdata:basics/structs.carbon.test FAILED in 0.7s
//toolchain/parser/testdata:basics/tuples.carbon.test FAILED in 0.6s
//toolchain/parser/testdata:basics/var.carbon.test FAILED in 0.7s
//toolchain/parser/testdata:for/fail_colon_instead_of_in.carbon.test FAILED in 0.7s
//toolchain/parser/testdata:for/fail_missing_in.carbon.test FAILED in 0.7s
//toolchain/parser/testdata:for/fail_missing_var.carbon.test FAILED in 0.7s
//toolchain/parser/testdata:for/nested.carbon.test FAILED in 0.6s
//toolchain/parser/testdata:for/simple.carbon.test FAILED in 0.8s
//toolchain/parser/testdata:function/definition/with_params.carbon.test FAILED in 0.8s
//toolchain/parser/testdata:operators/fixity_in_call.carbon.test FAILED in 0.7s
//toolchain/parser/testdata:operators/fixity_in_var.carbon.test FAILED in 0.8s
```
Co-authored-by: Chandler Carruth <chandlerc@gmail.com>