Allow parsing of mu (µ) character for durations (issue #8614) (#8647)

# Description
This is to resolve the issue
[8614](https://github.com/nushell/nushell/issues/8614).
It allows the parsing of the mu (µ) character for durations, so you can
type `10µs`, and it correctly outputs, whilst maintaining the current
`us` parsing as well.

It also forces `durations` to be entered in lower case. 


![image](https://user-images.githubusercontent.com/44570273/228217360-57ebc902-cec5-4683-910e-0b18fbe160b1.png)
(The bottom one `1sec | into duration --convert us` looks like an
existing bug, where converting to `us` outputs `us` rather than `µs`)

# User-Facing Changes

Allows the user to parse durations in µs
Forces `durations` to be entered in lower case rather than any case, and
will error if not in lower case.

# Tests + Formatting

Don't forget to add tests that cover your changes.

Make sure you've run and fixed any issues with these commands:

- `cargo fmt --all -- --check` to check standard code formatting (`cargo
fmt --all` applies these changes)
- `cargo clippy --workspace -- -D warnings -D clippy::unwrap_used -A
clippy::needless_collect` to check that you're using the standard code
style
- `cargo test --workspace` to check that all tests pass

> **Note**
> from `nushell` you can also use the `toolkit` as follows
> ```bash
> use toolkit.nu # or use an `env_change` hook to activate it
automatically
> toolkit check pr
> ```

# After Submitting

If your PR had any user-facing changes, update [the
documentation](https://github.com/nushell/nushell.github.io) after the
PR is merged, if necessary. This will help us keep the docs up to date.

---------

Co-authored-by: Stefan Holderbach <sholderbach@users.noreply.github.com>

crates/nu-parser/src/parser.rs

@@ -2604,24 +2604,32 @@ pub fn parse_duration_bytes(num_with_unit_bytes: &[u8], span: Span) -> Option<Ex
     }
 
     let num_with_unit = String::from_utf8_lossy(num_with_unit_bytes).to_string();
-    let uppercase_num_with_unit = num_with_unit.to_uppercase();
     let unit_groups = [
-        (Unit::Nanosecond, "NS", None),
-        (Unit::Microsecond, "US", Some((Unit::Nanosecond, 1000))),
-        (Unit::Millisecond, "MS", Some((Unit::Microsecond, 1000))),
-        (Unit::Second, "SEC", Some((Unit::Millisecond, 1000))),
-        (Unit::Minute, "MIN", Some((Unit::Second, 60))),
-        (Unit::Hour, "HR", Some((Unit::Minute, 60))),
-        (Unit::Day, "DAY", Some((Unit::Minute, 1440))),
-        (Unit::Week, "WK", Some((Unit::Day, 7))),
+        (Unit::Nanosecond, "ns", None),
+        (Unit::Microsecond, "us", Some((Unit::Nanosecond, 1000))),
+        (
+            // µ Micro Sign
+            Unit::Microsecond,
+            "\u{00B5}s",
+            Some((Unit::Nanosecond, 1000)),
+        ),
+        (
+            // μ Greek small letter Mu
+            Unit::Microsecond,
+            "\u{03BC}s",
+            Some((Unit::Nanosecond, 1000)),
+        ),
+        (Unit::Millisecond, "ms", Some((Unit::Microsecond, 1000))),
+        (Unit::Second, "sec", Some((Unit::Millisecond, 1000))),
+        (Unit::Minute, "min", Some((Unit::Second, 60))),
+        (Unit::Hour, "hr", Some((Unit::Minute, 60))),
+        (Unit::Day, "day", Some((Unit::Minute, 1440))),
+        (Unit::Week, "wk", Some((Unit::Day, 7))),
     ];
 
-    if let Some(unit) = unit_groups
-        .iter()
-        .find(|&x| uppercase_num_with_unit.ends_with(x.1))
-    {
+    if let Some(unit) = unit_groups.iter().find(|&x| num_with_unit.ends_with(x.1)) {
         let mut lhs = num_with_unit;
-        for _ in 0..unit.1.len() {
+        for _ in 0..unit.1.chars().count() {
             lhs.pop();
         }