add decimal to SyntaxShape (#7852)

# Description

This adds the `SyntaxShape::Decimal` so you can create custom commands
with `decimal` types such as:
```shell
def cmd [x:decimal] { echo $x }
```

/cc @kurokirasama

Internally this is a little messy since we have `Type::Float` and
`SyntaxShape::Decimal`. I originally named it `float` and
`SyntaxShape::Float` but since we have `into decimal` and `1.1 |
describe` reports `decimal`, I decided to change the SyntaxShape.

# User-Facing Changes

_(List of all changes that impact the user experience here. This helps
us keep track of breaking changes.)_

# 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

# 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.

crates/nu-parser/src/parser.rs

@@ -2758,6 +2758,7 @@ pub fn parse_shape_name(
         b"duration" => SyntaxShape::Duration,
         b"error" => SyntaxShape::Error,
         b"expr" => SyntaxShape::Expression,
+        b"float" | b"decimal" => SyntaxShape::Decimal,
         b"filesize" => SyntaxShape::Filesize,
         b"full-cell-path" => SyntaxShape::FullCellPath,
         b"glob" => SyntaxShape::GlobPattern,
@@ -2822,7 +2823,7 @@ pub fn parse_type(_working_set: &StateWorkingSet, bytes: &[u8]) -> Type {
         b"duration" => Type::Duration,
         b"error" => Type::Error,
         b"filesize" => Type::Filesize,
-        b"float" => Type::Float,
+        b"float" | b"decimal" => Type::Float,
         b"int" => Type::Int,
         b"list" => Type::List(Box::new(Type::Any)),
         b"number" => Type::Number,
@@ -4442,6 +4443,7 @@ pub fn parse_value(
             (expression, err)
         }
         SyntaxShape::Number => parse_number(bytes, span),
+        SyntaxShape::Decimal => parse_float(bytes, span),
         SyntaxShape::Int => parse_int(bytes, span),
         SyntaxShape::Duration => parse_duration(working_set, span),
         SyntaxShape::DateTime => parse_datetime(working_set, span),