Fix ShellCheck warnings and errors

[fpoli]

Fix warnings and errors reported by ShellCheck:

> shellcheck carbon.sh 

In carbon.sh line 23:
BASEDIR="$(realpath `dirname $0`)"
                    ^----------^ SC2046 (warning): Quote this to prevent word splitting.
                    ^----------^ SC2006 (style): Use $(...) notation instead of legacy backticks `...`.
                             ^-- SC2086 (info): Double quote to prevent globbing and word splitting.

Did you mean: 
BASEDIR="$(realpath $(dirname "$0"))"


In carbon.sh line 27:
if [ ! -f $CP_FILE ]; then
          ^------^ SC2086 (info): Double quote to prevent globbing and word splitting.

Did you mean: 
if [ ! -f "$CP_FILE" ]; then


In carbon.sh line 28:
    (cd $BASEDIR; sbt "export runtime:dependencyClasspath" | tail -n1 > $CP_FILE)
        ^------^ SC2086 (info): Double quote to prevent globbing and word splitting.
                                                                        ^------^ SC2086 (info): Double quote to prevent globbing and word splitting.

Did you mean: 
    (cd "$BASEDIR"; sbt "export runtime:dependencyClasspath" | tail -n1 > "$CP_FILE")


In carbon.sh line 31:
java -Xss30M -cp "`cat $CP_FILE`" viper.carbon.Carbon $@
                  ^------------^ SC2006 (style): Use $(...) notation instead of legacy backticks `...`.
                       ^------^ SC2086 (info): Double quote to prevent globbing and word splitting.
                                                      ^-- SC2068 (error): Double quote array expansions to avoid re-splitting elements.

Did you mean: 
java -Xss30M -cp "$(cat "$CP_FILE")" viper.carbon.Carbon $@

For more information:
  https://www.shellcheck.net/wiki/SC2068 -- Double quote array expansions to ...
  https://www.shellcheck.net/wiki/SC2046 -- Quote this to prevent word splitt...
  https://www.shellcheck.net/wiki/SC2086 -- Double quote to prevent globbing ...

[fpoli]

With this PR on Ubuntu 22.04 I can pass filenames containing spaces to Carbon. Before it was incorrectly complaining about extra arguments.

Note that the carbon.bat might benefit from similar fixes done to silicon.bat. See viperproject/silicon#588.

[gauravpartha]

Looks good to me, thanks!