Change the Rake task to one that uses namespaces as follows (previous task
names can also be used for compatibility).

| Previous Task  |    New Task    |
|----------------|----------------|
| api_doc        | doc:api        |
| capi_doc       | doc:capi       |
| clean_doc      | doc:clean      |
| clean_api_doc  | doc:clean:api  |
| clean_capi_doc | doc:clean:capi |
| view_api       | doc:view:api   |
| view_capi      | doc:view:capi  |
| gitlab_config  | gitlab:config  |
| gitlab_dockers | gitlab:dockers |

Fix line continuations with mixed indentation in squiggly heredocs

Merge pull request #5259 from shuujii/consider-the-case-a-local-variable-name-does-not-become-a-named-presym

Consider the case a local variable name does not become a named presym

Non-ASCII characters are allowed to local variable names, so they are not
always named presym.

Fix build with `enable_cxx_exception`

Take advantage of gembox

Merge branch 'improve-source-scanning-for-presym' of https://github.com/shuujii/mruby into shuujii-improve-source-scanning-for-presym

But you still cannot cross C function boundary.

Avoid 64-bit operations in `src/hash.c`; close #5201

Fix for escaped newlines in squiggly heredocs

replace ; to : of OPT_SETGV in codedump.c

Maybe it's a typo.

The idea of using `size_t` in `ea_next_capa_for` is by @dearblue.

Negative integer `>-65535` had wrong value, e,g, `p(-40550)` printed
`4294926746` since Nov. 2020, sigh.

Also avoid using `uint64_t`.

Fix regression for squiggly heredocs

Refine "wrong number of arguments" message in `mrb_get_args`

#### Before this patch:

```ruby
__send__         #=> wrong number of arguments
{}.default(1,2)  #=> wrong number of arguments
```

#### After this patch:

```ruby
__send__         #=> wrong number of arguments (given 0, expected 1+)
{}.default(1,2)  #=> wrong number of arguments (given 2, expected 0..1)
```

Update copyright year for 2021

Removed unusable `GC.test`