deprecate size to str size (#10772)

related to
-
https://discord.com/channels/601130461678272522/614613939334152217/1164530991931605062

# Description
it appears `size` is a command that operates on `string`s only and gives
the user information about the chars, graphemes and bytes of a string.
this looks like a command that should be a subcommand to `str` 😏 

this PR
- adds `str size`
- deprecates `size`

`size` is planned to be removed in 0.88

# User-Facing Changes
`str size` can be used for the same result as `size`.

# Tests + Formatting

# After Submitting
write a removal PR for `size`

crates/nu-command/src/default_context.rs

@@ -190,6 +190,7 @@ pub fn add_shell_command_context(mut engine_state: EngineState) -> EngineState {
             StrIndexOf,
             StrLength,
             StrReverse,
+            StrSize,
             StrStartsWith,
             StrSubstring,
             StrTrim,