tsc should give an error if "typings" in package.json is not a string

[DanielRosenwasser]

Courtesy of a long troubleshooting session with @kataras on September 16, 2015.

If you have "typings" in package.json as an array or something instead of a string, we will crash hard:

C:\Users\Makis-PC\AppData\Roaming\npm\node_modules\typescript\lib\tsc.js:483
if (path.charCodeAt(0) === 47) {
^
TypeError: path.charCodeAt is not a function
at getRootLength (C:\Users\Makis-PC\AppData\Roaming\npm\node_modules\typescript\lib\tsc.js:483:18)
at Object.combinePaths (C:\Users\Makis-PC\AppData\Roaming\npm\node_modules\typescript\lib\tsc.js:643:13)
at loadNodeModuleFromDirectory (C:\Users\Makis-PC\AppData\Roaming\npm\node_modules\typescript\lib\tsc.js:29439:73)
at loadModuleFromNodeModules (C:\Users\Makis-PC\AppData\Roaming\npm\node_modules\typescript\lib\tsc.js:29462:26)
at nodeModuleNameResolver (C:\Users\Makis-PC\AppData\Roaming\npm\node_modules\typescript\lib\tsc.js:29405:20)
at resolveModuleName (C:\Users\Makis-PC\AppData\Roaming\npm\node_modules\typescript\lib\tsc.js:29385:28)
at C:\Users\Makis-PC\AppData\Roaming\npm\node_modules\typescript\lib\tsc.js:29636:115
at Object.map (C:\Users\Makis-PC\AppData\Roaming\npm\node_modules\typescript\lib\tsc.js:155:29)
at resolveModuleNamesWorker (C:\Users\Makis-PC\AppData\Roaming\npm\node_modules\typescript\lib\tsc.js:29636:67)
at processImportedModules (C:\Users\Makis-PC\AppData\Roaming\npm\node_modules\typescript\lib\tsc.js:30013:35)

[mhegazy]

PRs are welcomed.