improve parsing of values with units (#9190)

closes #9111 

# Description

this pr improves parsing of values with units (`filesizes`, `durations`
and any other **future values**) by:

1. allowing underscores in the value part
```nu
> 42kb          # okay
> 42_sec        # okay
> 1_000_000mib  # okay
> 69k_b         # not okay, underscores not allowed in the unit
```

2. improving error messages involving these values
```nu
> sleep 40-sec

# before
Error: nu::parser::parse_mismatch

  × Parse mismatch during operation.
   ╭─[entry #42:1:1]
 1 │ sleep 40-sec
   ·       ──┬──
   ·         ╰── expected duration with valid units
   ╰────

# now
Error:
  × duration value must be a number
   ╭─[entry #41:1:1]
 1 │ sleep 40-sec
   ·       ─┬─
   ·        ╰── not a number
   ╰────
```

3. unifying parsing of these values. now all of these use one function

# User-Facing Changes

filesizes and durations can now have underscores for readability

crates/nu-parser/src/lib.rs

@@ -19,8 +19,8 @@ pub use lite_parser::{lite_parse, LiteBlock, LiteElement};
 pub use parse_keywords::*;
 
 pub use parser::{
-    is_math_expression_like, parse, parse_block, parse_duration_bytes, parse_expression,
-    parse_external_call, trim_quotes, trim_quotes_str, unescape_unquote_string,
+    is_math_expression_like, parse, parse_block, parse_expression, parse_external_call,
+    parse_unit_value, trim_quotes, trim_quotes_str, unescape_unquote_string, DURATION_UNIT_GROUPS,
 };
 
 #[cfg(feature = "plugin")]