[gnoweb] Allow -send value to be set via txlink

[leohhhn]

Description

We should have a way to set a -send amount for via txlink, which would be reflected in gnokey commands in the help page, as well as Adena.

While this is a practical feature, it does pose a certain threat of scamming - this is why we need to make sure that when such a link is clicked, it clearly shows the user what it's doing.

For gnokey help commands, we could highlight the -send flag and the amount in red so that it pops out, or put a warning banner above, while also adding a text field that would allow the user to modify the amount (& denom!) in gnoweb directly, without going near the CLI.

For Adena, we could do something similar.

A suggested way to set this parameter in gnoweb links is .send, as it doesn't conflict with any possible Gno function arguments (function arg names cannot start with .).

Example: $help&func=MyFunc&arg=whatever&.send=10000ugnot

We also need to modify txlink & helplink libraries to match this down the line.

cc @alexiscolin @moul @dongwon8247

[moul]

i suggest not only having a warning, but having an extra action to do, like clicking to apply the suggested amount of sent token.

[moul]

Just a note for those coming here: I suggest using special arguments with a dot prefix, as they cannot be the variable name of a function. This approach eliminates the risk of matching an actual variable. For example, we can have &send=foo&.send=1999ugnot, which is weird but prevents any mismatch.
Alternatively, we could use another prefix that cannot exist in a variable name. An underscore (_) might be a good option since it is more visually distinct than a dot. However, I'm unsure if we can use it in functions in Go as well. The goal is to find a "special" prefix that avoids any mismatch while remaining visually clear. I'm open to suggestions.