proposal-typescript: change algo how .js .d.ts gets handled in the ide

[frank-dspeed]

At present in case of .js as authoring format the .js is authoritativ while using allowJs checkJs
when not the d.ts gets used

i propose a mixed mode where allowJs checkJs + d.ts gets used when editing the .js file and also error on divergence inside .js and .d.ts the d.ts is authoritativ for none existing values in the js while the js is autoritativ for the case when it not exists in .d.ts