Rename methods on main api

[dsherret]

Add:

• createSourceFile(filePath: string): SourceFile; -- Sometimes it's nice to just create an empty source file.

Change:

• getOrAddSourceFile(filePath: string): SourceFile; -> addExistingSourceFile(filePath: string): SourceFile; - Should say "existing" to make it clear that this is an existing source file from the disk.
• addSourceFiles(...fileGlobs: string[]): void -> addExistingSourceFiles(...fileGlobs: string[]): SourceFile[] -- Return the source files that matched
• addSourceFileFromText(filePath: string, sourceFileText: string): SourceFile; -> createSourceFile(filePath: string, sourceFileText: string): SourceFile;
• addSourceFileFromStructure(filePath: string, structure: SourceFileStructure): SourceFile; -> createSourceFile(filePath: string, structure: SourceFileStructure): SourceFile;