Add, move commands in shell syntax file

[niten94]

The commands below are added in this pull request:

• break
• command
• continue
• eval
• exec - There are times the command is used with replacing the shell process and other commands in a script are not run when it is replaced, so it is added in the "Conditionals and control flow" group.
• getopt
• getopts
• trap - It is usually used with running a command when a signal is received so it is added in the "Conditionals and control flow" group.
• wait - The command returns when commands in background have finished running so it is added in the "Conditionals and control flow" group.

The commands below are highlighted using a different color group in this pull request:

• local
• read
• shift
• time

The commands that are added and moved except getopt are usually builtin commands in sh implementations so they can be looked up in reference pages of those implementations.

[niten94]

eval was highlighted as a keyword because commands like exit can be run using it, but such commands are not usually run using eval so I have moved it to "Shell commands". I was also thinking about the same with command so the note about command in the pull request description has been removed.

The pull request description has also been edited so that it would be a bit more clear why some commands were added in "Conditionals and control flow".

[dmaluka]

LGTM. @JoeKar WDYT?