fix: Properly append a $ suffix to reserved words in index files

[hristozov]

Problem

See #383.

Long story short, exports that use reserved keywords (e.g. import) in index.ts files (when "indexFile": true) do not have the $ prefix used everywhere else in the emitted code. As a result, the index.ts would reference import which is import$ everywhere else. This leads to TS code that does not compile.

Solution

The ensureNotReserved helper applies the $ suffix. It's already applieds on the methodName to produce importName. However, the index.ts template references methodName and not the importName.

This change switches the template to importName.

Also wrote a test for the problematic case, that fails without the current fix. To use the generated functions in the test, I had to expose them from ng-openapi-gen.ts.

[luisfpg]

I'm sorry, but I went through the open issues first, and ended up fixing #383 without noticing this PR...
Anyway, another issue (#389) changed again this fix, because it needed additional work.
Thanks for the PR, anyway.