Profile: fix heap snapshot is valid char check (#53984)

Followup to #53833
Fixes a failure seen in #53974
(below)

I believe this is the more correct check to make?

The heapsnapshot generated from this PR is viewable in vscode.

```
2024-04-06 09:33:58 EDT	      From worker 7:	ERROR: Base.InvalidCharError{Char}('\xc1\xae')
2024-04-06 09:33:58 EDT	      From worker 7:	Stacktrace:
2024-04-06 09:33:58 EDT	      From worker 7:	  [1] throw_invalid_char(c::Char)
2024-04-06 09:33:58 EDT	      From worker 7:	    @ Base ./char.jl:86
2024-04-06 09:33:58 EDT	      From worker 7:	  [2] UInt32
2024-04-06 09:33:58 EDT	      From worker 7:	    @ ./char.jl:133 [inlined]
2024-04-06 09:33:58 EDT	      From worker 7:	  [3] category_code
2024-04-06 09:33:58 EDT	      From worker 7:	    @ ./strings/unicode.jl:339 [inlined]
2024-04-06 09:33:58 EDT	      From worker 7:	  [4] isassigned
2024-04-06 09:33:58 EDT	      From worker 7:	    @ ./strings/unicode.jl:355 [inlined]
2024-04-06 09:33:58 EDT	      From worker 7:	  [5] isassigned
2024-04-06 09:33:58 EDT	      From worker 7:	    @ /cache/build/tester-amdci5-14/julialang/julia-master/julia-41d026beaf/share/julia/stdlib/v1.12/Unicode/src/Unicode.jl:138 [inlined]
2024-04-06 09:33:58 EDT	      From worker 7:	  [6] print_str_escape_json(stream::IOStream, s::String)
2024-04-06 09:33:58 EDT	      From worker 7:	    @ Profile.HeapSnapshot /cache/build/tester-amdci5-14/julialang/julia-master/julia-41d026beaf/share/julia/stdlib/v1.12/Profile/src/heapsnapshot_reassemble.jl:239
2024-04-06 09:33:59 EDT	      From worker 7:	  [7] (::Profile.HeapSnapshot.var"#5#6"{IOStream})(strings_io::IOStream)
2024-04-06 09:33:59 EDT	      From worker 7:	    @ Profile.HeapSnapshot /cache/build/tester-amdci5-14/julialang/julia-master/julia-41d026beaf/share/julia/stdlib/v1.12/Profile/src/heapsnapshot_reassemble.jl:192
```

Author: IanButterworth

stdlib/Manifest.toml

@@ -191,7 +191,6 @@ uuid = "de0858da-6303-5e67-8744-51eddeeeb8d7"
 version = "1.11.0"
 
 [[deps.Profile]]
-deps = ["Unicode"]
 uuid = "9abbd945-dff8-562f-b5e8-e1ebf5ef1b79"
 version = "1.11.0"
 

stdlib/Profile/Project.toml

@@ -2,12 +2,6 @@ name = "Profile"
 uuid = "9abbd945-dff8-562f-b5e8-e1ebf5ef1b79"
 version = "1.11.0"
 
-[deps]
-Unicode = "4ec0a83e-493e-50e2-b9ac-8f72acf5a8f5"
-
-[compat]
-Unicode = "1.11.0"
-
 [extras]
 Base64 = "2a0f44e3-6c83-55bd-87e4-b1978d98bd5f"
 Logging = "56ddb016-857b-54e1-b83d-db4d58db5568"

stdlib/Profile/src/heapsnapshot_reassemble.jl

@@ -2,8 +2,6 @@
 
 module HeapSnapshot
 
-using Unicode
-
 """
     assemble_snapshot(filepath::AbstractString, out_file::AbstractString)
 
@@ -236,7 +234,7 @@ function print_str_escape_json(stream::IO, s::AbstractString)
             print(stream, "\\t")
         elseif '\x00' <= c <= '\x1f'
             print(stream, "\\u", lpad(string(UInt16(c), base=16), 4, '0'))
-        elseif !Unicode.isassigned(c)
+        elseif !isvalid(c)
             # we have to do this because vscode's viewer doesn't like the replace character
             print(stream, "[invalid unicode character]")
         else