Fix: missing parse error when extra tokens are given to let bindings (#12238)

Manual checks are added to `parse_let`, `parse_mut`, and `parse_const`.
`parse_var_with_opt_type` is also fixed to update `spans_idx` correctly.
Fixes #12125.

It's technically a fix, but I'd rather not merge this directly. I'm
making this PR to bring into attention the code quality of the parser
code. For example:

* Inconsistent usage of `spans_idx`. What is its purpose, and which
parsing functions need it? I suspect it's possible to remove the usage
of `spans_idx` entirely.
* Lacking documentation for top-level functions. What does `mutable`
mean for `parse_var_with_opt_type()`?
* Inconsistent error reporting. Usage of both `working_set.error()` and
`working_set.parse_errors.push()`. Using `ParseError::Expected` for an
invalid variable name when there's `ParseError::VariableNotValid` (from
`parser.rs:5237`). Checking variable names manually when there's
`is_variable()` (from `parser.rs:2905`).
* `span()` is a terrible name for a function that flattens a bunch of
spans into one (from `nu-protocal/src/span.rs:92`). The top-level
comment (`Used when you have a slice of spans of at least size 1`)
doesn't help either.

I've only looked at a small portion of the parser code; I expect there
are a lot more. These issues made it much harder to fix a simple bug
like #12125. I believe we should invest some effort to cleanup the
parser code, which will ease maintainance in the future. I'll willing to
help if there is interest.

tests/parsing/mod.rs

@@ -331,3 +331,21 @@ fn parse_let_signature(#[case] phrase: &str) {
     let actual = nu!(phrase);
     assert!(actual.err.is_empty());
 }
+
+#[test]
+fn parse_let_signature_missing_colon() {
+    let actual = nu!("let a int = 1");
+    assert!(actual.err.contains("nu::parser::extra_tokens"));
+}
+
+#[test]
+fn parse_mut_signature_missing_colon() {
+    let actual = nu!("mut a record<a: int b: int> = {a: 1 b: 1}");
+    assert!(actual.err.contains("nu::parser::extra_tokens"));
+}
+
+#[test]
+fn parse_const_signature_missing_colon() {
+    let actual = nu!("const a string = 'Hello World\n'");
+    assert!(actual.err.contains("nu::parser::extra_tokens"));
+}