Update internal use of decimal to float (#10333)

# Description
We made the decision that our floating point type should be referred to
as `float` over `decimal`.
Commands were updated by #9979 and #10320

Now make the internal codebase consistent in referring to this data type
as `float`.

Work for #10332

# User-Facing Changes

`decimal` has been removed as a type name/symbol. 

Instead of 
```nushell
def foo [bar: decimal] decimal -> decimal {}
```
use 
```nushell
def foo [bar: float] float -> float {}
```

Potential effect of `SyntaxShape`'s `Display` implementation now also
referring to `float` instead of `decimal`

# Details
- Rename `SyntaxShape::Decimal` to `Float`
- Update `Display for SyntaxShape` to `float`
- Update error message + fn name in dataframe code
- Fix docs in command examples
- Rename tests that are float specific
- Update doccomment on `SyntaxShape`
- Update comment in script

# Tests + Formatting
Updates the names of some tests

crates/nu-parser/src/parser.rs

@@ -2709,7 +2709,7 @@ pub fn parse_shape_name(
         b"duration" => SyntaxShape::Duration,
         b"error" => SyntaxShape::Error,
         b"expr" => SyntaxShape::Expression,
-        b"float" | b"decimal" => SyntaxShape::Decimal,
+        b"float" => SyntaxShape::Float,
         b"filesize" => SyntaxShape::Filesize,
         b"full-cell-path" => SyntaxShape::FullCellPath,
         b"glob" => SyntaxShape::GlobPattern,
@@ -4654,7 +4654,7 @@ pub fn parse_value(
             expression
         }
         SyntaxShape::Number => parse_number(working_set, span),
-        SyntaxShape::Decimal => parse_float(working_set, span),
+        SyntaxShape::Float => parse_float(working_set, span),
         SyntaxShape::Int => parse_int(working_set, span),
         SyntaxShape::Duration => parse_duration(working_set, span),
         SyntaxShape::DateTime => parse_datetime(working_set, span),