fix: improve Fish shell support (and Cshell) (#1429)

I faced the same issue mentioned in #1299, this PR fixes the issue by
taking in account specific `set` and `unset` syntax for Fish shell.

The post install instruction (i.e : `./emsdk activate latest`) provides
wrong instructions in Fish shell and Cshell cases.
This should be fixed by this PR too, although the code is a bit
redundant and it could be more cleaner.

emsdk_env.fish

@@ -3,6 +3,8 @@
 #Now, when you want to use the SDK, run this alias first to set up
 #your environment.
 
+set -gx EMSDK_FISH 1
+
 set -l script (status -f)
 set -l dir (dirname $script)