Replace :! binding syntax with phase keywords and contextual defaults (#7479)

Implement the toolchain side of proposal #7254, removing the `:!`
binding
syntax for generic and template parameters in favor of the keywords
`generic`,
`template`, and `runtime` plus contextual defaults for phase.

For valid programs this is semantics-preserving: each binding resolves
to the
same phase, and produces the same SemIR, as it did under `:!`/`:`. The
parser
derives a binding's phase from its syntactic context plus any explicit
phase
keyword; new diagnostics and error recovery for misused keywords are
described
below.

Implementation details for each component:

- Lexer: remove the `:!` (`ColonExclaim`) token, move its virtual
parse-node
  budget onto `:`, and add the `generic` and `runtime` keywords.
- Parser: thread a `BindingContext` (`ExplicitParam`, `DeducedParam`, or
`CompileTimeEntityParam`) from declaration introducers down through
parameter
lists to each binding pattern, using a one-token lookahead to
distinguish a
name-qualifier parameter list from a declaration's own final list.
Parameters
of a compile-time entity (`class`, `interface`, `constraint`, `choice`,
`alias`, `export`, `namespace`) and deduced `[]` parameters default to
checked
generic; explicit function parameters and local bindings default to
runtime.
`HandleBindingPattern` resolves the phase from that context plus the
keyword: a
`generic` keyword needs no node of its own (the phase is carried by the
  binding's node kind), while a `runtime` keyword is preserved as a
`RuntimeBindingName` node so `check` can name it in a diagnostic. A
phase
keyword that is merely redundant with the contextual default is
diagnosed
  here, without invalidating the parse tree.
- Check: a phase keyword that is invalid for its context (for example
`runtime`
on a checked-generic parameter) is diagnosed here, and recovers by
building an
error binding that still introduces the name so that later uses of it do
not
  produce cascading errors.

The removed `:!` syntax is now rejected as an ordinary parse error.

The `form`/`:?`/`->?` ("extended types") portion of proposal #7254 is
left for a
separate change.

Assisted-by: Claude Code

toolchain/check/decl_name_stack.h

@@ -18,8 +18,8 @@ class Context;
 // Provides support and stacking for qualified declaration name handling.
 //
 // A qualified declaration name will consist of entries, which are `Name`s
-// optionally followed by generic parameter lists, such as `Vector(T:! type)`
-// in `fn Vector(T:! type).Clear();`, but parameter lists aren't supported yet.
+// optionally followed by generic parameter lists, such as `Vector(T: type)`
+// in `fn Vector(T: type).Clear();`, but parameter lists aren't supported yet.
 // Identifiers such as `Clear` will be resolved to a name if possible, for
 // example when declaring things that are in a non-generic type or namespace,
 // and are otherwise marked as an unresolved identifier.
@@ -36,7 +36,7 @@ class Context;
 // appearing later in the declaration name itself. For example, in:
 //
 // ```
-// fn ClassA.ClassB(T:! U).Fn() { var x: V; }
+// fn ClassA.ClassB(T: U).Fn() { var x: V; }
 // ```
 //
 // the lookup for `U` looks in `ClassA`; the lookup for `V` looks first in