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-command/tests/commands/str_/into_string.rs

@@ -21,7 +21,7 @@ fn from_number() {
 }
 
 #[test]
-fn from_decimal() {
+fn from_float() {
     let actual = nu!(r#"
         echo 1.5 | into string
         "#);
@@ -90,7 +90,7 @@ fn from_filesize() {
 }
 
 #[test]
-fn from_decimal_correct_trailing_zeros() {
+fn from_float_correct_trailing_zeros() {
     let actual = nu!(r#"
         1.23000 | into string -d 3
         "#);
@@ -99,7 +99,7 @@ fn from_decimal_correct_trailing_zeros() {
 }
 
 #[test]
-fn from_int_decimal_correct_trailing_zeros() {
+fn from_int_float_correct_trailing_zeros() {
     let actual = nu!(r#"
         1.00000 | into string -d 3
         "#);
@@ -108,7 +108,7 @@ fn from_int_decimal_correct_trailing_zeros() {
 }
 
 #[test]
-fn from_int_decimal_trim_trailing_zeros() {
+fn from_int_float_trim_trailing_zeros() {
     let actual = nu!(r#"
         1.00000 | into string | $"($in) flat"
         "#);