Update syn requirement from 1.0.107 to 2.0.48 (#881)

* Update syn requirement from 1.0.107 to 2.0.48

Updates the requirements on [syn](https://github.com/dtolnay/syn) to permit the latest version.
- [Release notes](https://github.com/dtolnay/syn/releases)
- [Commits](https://github.com/dtolnay/syn/compare/1.0.107...2.0.48)

---
updated-dependencies:
- dependency-name: syn
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <support@github.com>

* rtic-macros: attr.path -> attr.path()

* rtic-macros: tokens -> parse_args()

Fix parsing of InitArgs, IdleArgs, *Args

Including HardwareTaskArgs, SoftwareTaskArgs

* rtic-macros: Rename content to input

As syn2 removes the surrounding parenthesis as
part of parse_args() the distinction between
input and content is redundant

* rtic-macros: Handle removal of Expr::Type

Manually parse local_resources

With type ascription de-RFCd syn2 dropped
Expr::Type

* rtic-macros: Syn upgrade CHANGELOG

* rtic-macro: Retain most old errors as they were

Spans are not equal, but good enough

* rtic-macros: syn2 changed some error messages

Additionally some spans were not retained
with the manual parsing workaround

* rtic-macros: clippy fixes

---------

Signed-off-by: dependabot[bot] <support@github.com>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>
Co-authored-by: Henrik Tjäder <henrik@tjaders.com>

rtic-macros/ui/local-malformed-1.stderr

@@ -1,4 +1,4 @@
-error: unexpected end of input, expected one of: `for`, parentheses, `fn`, `unsafe`, `extern`, identifier, `::`, `<`, square brackets, `*`, `&`, `!`, `impl`, `_`, lifetime
+error: unexpected end of input, expected one of: `for`, parentheses, `fn`, `unsafe`, `extern`, identifier, `::`, `<`, `dyn`, square brackets, `*`, `&`, `!`, `impl`, `_`, lifetime
   --> ui/local-malformed-1.rs:11:23
    |
 11 |     #[task(local = [a:])]

rtic-macros/ui/local-malformed-2.stderr

@@ -2,4 +2,4 @@ error: malformed, expected 'IDENT: TYPE = EXPR'
   --> ui/local-malformed-2.rs:11:21
    |
 11 |     #[task(local = [a: u32])]
-   |                     ^^^^^^
+   |                     ^

rtic-macros/ui/local-malformed-3.stderr

@@ -1,4 +1,4 @@
-error: unexpected end of input, expected expression
+error: unexpected end of input, expected an expression
   --> ui/local-malformed-3.rs:11:29
    |
 11 |     #[task(local = [a: u32 =])]

rtic-macros/ui/local-shared-attribute.stderr

@@ -1,6 +1,5 @@
 error: attributes are not supported here
-  --> ui/local-shared-attribute.rs:17:9
+  --> ui/local-shared-attribute.rs:18:9
    |
-17 | /         #[test]
-18 | |         b, // Error
-   | |_________^
+18 |         b, // Error
+   |         ^