Add script commands to generate some client <TAGS>

[guilherme-gm]

Pull Request Prelude

• I have followed proper Hercules code styling.
• I have read and understood the contribution guidelines before making this PR.
• I am aware that this PR may be closed if the above-mentioned criteria are not fulfilled.

Changes Proposed

This PR adds some script commands which generates strings for client tags:

• mesnavigation: General purpose command for <NAVI>;
• mesmobspawn: Special command for <NAVI> that opens a window "searching" for a monster -- This is a special behavior in navigation
• mesurl: For <URL> tag
• mestipbox: For <TIPBOX> tag

Using those commands takes into account client support and tries to fall back to some sensible results when something is not supported.

I decided to make them as script commands instead of Global Functions because:

1. We keep client-specific logic into clif -- I feel like this is too client specific to be handled by scripts
2. We can eventually use these from source directly if there is a need

Issues addressed: None, I think

[guilherme-gm]

I thought about making a mesmapnavi("label", "mapname") and mesnpcnavi("NPC Name"{, "label"}) which could check whether map or npc exists. But I don't remember seeing those being used in official scripts... so I am not sure if it is worth it.