Lex Jupyter line magic with `Mode::Jupyter`
This PR adds a new token `MagicCommand`[^1] which the lexer will
recognize when in `Mode::Jupyter`. The rules for the lexer is as
follows:
1. Given that we are at the start of line, skip the indentation and look
for [characters that represent the start of a magic
command](635815e8f1/IPython/core/inputtransformer2.py (L335-L346)),
determine the magic kind and capture all the characters following it as
the command string.
2. If the command extends multiple lines, the lexer will skip the line
continuation character (`\`) but only if it's followed by a newline
(`\n` or `\r`). The reason to skip this only in case of newline is
because they can occur in the command string which we should not skip:
```rust
// Skip this backslash
// v
// !pwd \
// && ls -a | sed 's/^/\\ /'
// ^^
// Don't skip these backslashes
```
3. The parser, when in `Mode::Jupyter`, will filter these tokens before
the parsing begins. There is a small caveat when the magic command is
indented. In the following example, when the parser filters out magic
command, it'll throw an indentation error:
```python
for i in range(5):
!ls
# What the parser will see
for i in range(5):
```
[^1]: I would prefer to have some other name as this not only represent
a line magic (`%`) but also shell command (`!`), help command (`?`) and
others. In original implementation, it's named as ["IPython
Syntax"](635815e8f1/IPython/core/inputtransformer2.py (L332))
3b4c8fffe5