Plugin explicit flags (#11581)

# Description
#11492 fixed flags for builtin commands but I missed that plugins don't
use the same `has_flag` that builtins do. This PR addresses this.

Unfortunately this means that return value of `has_flag` needs to change
from `bool` to `Result<bool, ShellError>` to produce an error when
explicit value is not a boolean (just like in case of `has_flag` for
builtin commands. It is not possible to check this in
`EvaluatedCall::try_from_call` because

# User-Facing Changes
Passing explicit values to flags of plugin commands (like `--flag=true`
`--flag=false`) should work now.
BREAKING: changed return value of `EvaluatedCall::has_flag` method from
`bool` to `Result<bool, ShellError>`

# Tests + Formatting
Added tests and updated documentation and examples

crates/nu_plugin_example/src/example.rs

@@ -40,7 +40,7 @@ impl Example {
         // Keep this in mind when designing your plugin signatures
         let a: i64 = call.req(0)?;
         let b: String = call.req(1)?;
-        let flag = call.has_flag("flag");
+        let flag = call.has_flag("flag")?;
         let opt: Option<i64> = call.opt(2)?;
         let named: Option<String> = call.get_flag("named")?;
         let rest: Vec<String> = call.rest(3)?;