Fix deno install command name including dot on Windows

[yuta0801]

The path.with_extension()'s arg expect just file extension not include dots like cmd
So passing extension starting with a dot, it returns a file path containing two dots like file..cmd
This means all commands installed with demo install end with a dot like command. on Windows

Current deno install behavior on Windows (log)

PS D:\> deno install --allow-net --allow-read file_server https://deno.land/std/http/file_server.ts
Download https://deno.land/std/http/file_server.ts
Compile https://deno.land/std/http/file_server.ts
Download https://deno.land/std/path/mod.ts
Download https://deno.land/std/http/server.ts
Download https://deno.land/std/flags/mod.ts
Download https://deno.land/std/testing/asserts.ts
Download https://deno.land/std/http/io.ts
Download https://deno.land/std/path/win32.ts
Download https://deno.land/std/path/posix.ts
Download https://deno.land/std/path/constants.ts
Download https://deno.land/std/path/constants.ts
Download https://deno.land/std/path/interface.ts
Download https://deno.land/std/path/glob.ts
Download https://deno.land/std/path/globrex.ts
Download https://deno.land/std/path/utils.ts
Download https://deno.land/std/fmt/colors.ts
Download https://deno.land/std/testing/diff.ts
Download https://deno.land/std/testing/format.ts
Download https://deno.land/std/io/bufio.ts
Download https://deno.land/std/util/async.ts
Download https://deno.land/std/strings/mod.ts
Download https://deno.land/std/io/util.ts
Download https://deno.land/std/strings/encode.ts
Download https://deno.land/std/strings/decode.ts
Download https://deno.land/std/strings/pad.ts
Download https://deno.land/std/textproto/mod.ts
Download https://deno.land/std/http/http_status.ts
✅ Successfully installed file_server
C:\Users\yuta\.deno\bin\file_server..cmd
PS D:\> file_server
file_server : The term 'file_server' is not recognized as the name of a cmdlet, function, script file, or operable program.
Check the spelling of the name, or if a path was included, verify that the path is correct and try again.
At line:1 char:1
+ file_server
+ ~~~~~~~~~~~
+ CategoryInfo          : ObjectNotFound: (file_server:String) [], CommandNotFoundException
+ FullyQualifiedErrorId : CommandNotFoundException
PS D:> file_server.
D:>deno.exe "run" "--allow-read" "--allow-net" "https://deno.land/std/http/file_server.ts"

HTTP server listening on http:https://0.0.0.0:4500/

^C
PS D:> file_server..cmd

D:>deno.exe "run" "--allow-read" "--allow-net" "https://deno.land/std/http/file_server.ts"

HTTP server listening on http:https://0.0.0.0:4500/

^C

[CLAassistant]

All committers have signed the CLA.

[piscisaureus]

LGTM, thanks!