have `gleam check`, `gleam build` validate the gleam.toml schema

Currently the inclusion of a spurious field in gleam.toml goes silently unremarked upon.

This caused me some grief just now as I didn't spot that a local dependency list should appear under [dependencies] not above it.

The proposal would be for gleam build to report the presence of unrecognized/unsupported fields in gleam.toml before it tries to build anything, i.e., for gleam build to validate the gleam.toml schema. (As versioned by the Gleam version.)