Fix unplugin emitDeclaration and Windows behavior

[edemaine]

Implement @imm's suggestions for fixing emitDeclaration:

• Remove mkdir preceding this.emitFile; this.emitFile should do this already (indeed, unplugin does it explicitly for esbuild)
• Switch emit call from async to synchronous.
• Add warning when using esbuild and no outdir option (as our example was doing), which prevents us (technically, unplugin) from emitting declarations because don't know where to put them.

Additionally, I realized there were bugs specific to Windows, which I was seeing but @imm wasn't (presumably not being on Windows):

• fsMap was getting the wrong code for slashed versions of filenames: the Civet source instead of the compiled TS! So I was getting error messages like main.civet.jsx:1:5 - error TS1434: Unexpected keyword or identifier., despite things seeming to also work (but probably not very well).
• this.emitFile was getting called on both original and slashed versions of files. This was leading to double emits, with error messages like (!) The emitted file "main.civet.d.ts" overwrites a previously emitted file of the same name. To fix this, I removed the duplicates from fsMap in the emit phase.

I've tested esbuild and rollup and this seems to work much better.