Make file saving auto-adapt more to a language's extension

[mflatt]

For Windows, move a "*." pattern to the front when it matches the default extension, because put-file uses that first pattern to determine the default file extension.

For example, for a file that starts #lang rhombus, the default extension for saving a file turns into ".rhm" instead of ".rkt".

[rfindler]

I was confused by the fancy string munging at first but now I see how it works.

LGTM.