Update typings in model.ts to allow nulls

[rcjsuen]

The searching functions in ITextModel allow one of its parameters to be null but the typings do not have this declared. The parameter is defined as string instead of string | null. If someone is compiling code in strict null checking mode then a false positive error will be flagged.

See microsoft/monaco-editor#580.

[rcjsuen]

Looks like it's no good. I'll try a new fix later.

[09:38:16] Error: monaco.d.ts is no longer up to date. Please run gulp watch and commit the new file.
    at formatError (C:\projects\vscode\node_modules\gulp\bin\gulp.js:169:10)
    at Gulp.<anonymous> (C:\projects\vscode\node_modules\gulp\bin\gulp.js:195:15)
    at emitOne (events.js:116:13)
    at Gulp.emit (events.js:211:7)
    at Gulp.Orchestrator._emitTaskDone (C:\projects\vscode\node_modules\orchestrator\index.js:264:8)
    at C:\projects\vscode\node_modules\orchestrator\index.js:275:23
    at finish (C:\projects\vscode\node_modules\orchestrator\lib\runTask.js:21:8)
    at Stream.<anonymous> (C:\projects\vscode\node_modules\orchestrator\lib\runTask.js:52:4)
    at Stream.f (C:\projects\vscode\node_modules\orchestrator\node_modules\once\once.js:17:25)
    at emitOne (events.js:121:20)
    at Stream.emit (events.js:211:7)
    at run (C:\projects\vscode\build\lib\compilation.js:124:30)
    at setInputFile (C:\projects\vscode\build\lib\compilation.js:114:13)
    at Stream.<anonymous> (C:\projects\vscode\build\lib\compilation.js:137:13)
    at Stream.stream.write (C:\projects\vscode\node_modules\through\index.js:26:11)
    at Stream.ondata (internal/streams/legacy.js:16:26)
[09:38:16] Finished compilation with 0 errors after 86207 ms

[alexdima]

@rcjsuen We are generating the file monaco.d.ts from the sources, and then we are including it in the repository, so when you compile vscode locally, you have to check that file in too.

[rcjsuen]

Thank you for finishing this up for me, @alexandrudima!