Improve error messages for raw strings (#60762)

This diff improves error messages around raw strings in a few ways:
- Catch extra trailing `#` in the parser. This can't be handled in the lexer because we could be in a macro that actually expects another # (see test)
- Refactor & unify error handling in the lexer between ByteStrings and RawByteStrings
- Detect potentially intended terminators (longest sequence of "#*" is suggested)

src/test/ui/parser/raw/raw-byte-string-eof.stderr

@@ -2,7 +2,9 @@ error[E0748]: unterminated raw string
   --> $DIR/raw-byte-string-eof.rs:2:5
    |
 LL |     br##"a"#;
-   |     ^ unterminated raw string
+   |     ^      - help: you might have intended to terminate the string here: `##`
+   |     |
+   |     unterminated raw string
    |
    = note: this raw string should be terminated with `"##`
 

src/test/ui/parser/raw/raw-str-in-macro-call.rs

@@ -0,0 +1,14 @@
+// check-pass
+
+macro_rules! m1 {
+    ($tt:tt #) => ()
+}
+
+macro_rules! m2 {
+    ($tt:tt) => ()
+}
+
+fn main() {
+    m1!(r#"abc"##);
+    m2!(r#"abc"#);
+}

src/test/ui/parser/raw/raw-str-unbalanced.rs

@@ -1,4 +1,4 @@
 static s: &'static str =
     r#"
-      "## //~ ERROR expected one of `.`, `;`, `?`, or an operator, found `#`
+      "## //~ too many `#` when terminating raw string
 ;

src/test/ui/parser/raw/raw-str-unbalanced.stderr

@@ -1,8 +1,10 @@
-error: expected one of `.`, `;`, `?`, or an operator, found `#`
+error: too many `#` when terminating raw string
   --> $DIR/raw-str-unbalanced.rs:3:9
    |
 LL |       "##
-   |         ^ expected one of `.`, `;`, `?`, or an operator
+   |         ^ help: Remove the extra `#`
+   |
+   = note: The raw string started with 1 `#`s
 
 error: aborting due to previous error
 

src/test/ui/parser/raw/raw_string.stderr

@@ -2,7 +2,9 @@ error[E0748]: unterminated raw string
   --> $DIR/raw_string.rs:2:13
    |
 LL |     let x = r##"lol"#;
-   |             ^ unterminated raw string
+   |             ^       - help: you might have intended to terminate the string here: `##`
+   |             |
+   |             unterminated raw string
    |
    = note: this raw string should be terminated with `"##`