feat: add positional argument support to slash commands

[Rienkim]

• feat: implement $ARGUMENTS indexing and splitting for /commands #2199 had a merge conflict that was not resolved, so I am submitting a new PR with the merge conflict resolved.
• Removing $ARGUMENTS[n+], $ARGUMENTS[n++] becasue they can be replaced by $ARGUMENTS[n:n+1], $ARGUMENTS[n:n+2].
• You can still use $ARGUMENTS as before.

Closes #2199

[rekram1-node]

@Rienkim I think we are gonna do named args instead of that format, if you are up for it if you update this pr to support named args I will review and merge it

[Rienkim]

@rekram1-node
Is it okay if the named argument has the following format?
/mycommand test.ts --mode=strict --env=production

In the custom command prompt, ARGUMENTS is applied as follows:

$ARGUMENTS[0]     // test.ts
$ARGUMENTS[1]     // --mode=strict
$ARGUMENTS[2]     // --env=production
$ARGUMENTS        // "test.ts --mode=strict --env=production"
$ARGUMENTS.mode   // "strict"
$ARGUMENTS.env    // "production"

[rekram1-node]

@Rienkim actually I talked with the team let's just do a similar thing to what claude code does,

we can use the $1, $2 for positional args like sql, and otherwise everything is replaced into $ARGUMENTS

See their docs for inspiration: https://docs.claude.com/en/docs/claude-code/slash-commands#arguments

[Rienkim]

@rekram1-node
I agree with your suggestion. I will change code of this PR to support $1, $2 for positional args.

[rekram1-node]

sweet thanks!

[Rienkim]

@rekram1-node
I updated code with $1, $2 for positional args.

[rekram1-node]

Nice work! Gonna make some tweaks and merge