ARROW-15712 [R] Add a type method for Expression objects

[dragosmg]

This would allow for more consistent syntax when extracting the type of an expression.
A block like this:

if (inherits(x, "Expression")) { 
class <- x$type()$ToString() 
} else { 
class <- type(x)$ToString() 
}

would be simplified to

class <- type(x)$ToString()

[github-actions]

https://issues.apache.org/jira/browse/ARROW-15712

[thisisnic]

This is great!

[ursabot]

Benchmark runs are scheduled for baseline = 4a439c2 and contender = cd20efb. cd20efb is a master commit associated with this PR. Results will be available as each benchmark for each run completes.
Conbench compare runs links:
[Finished ⬇️0.0% ⬆️0.0%] ec2-t3-xlarge-us-east-2
[Finished ⬇️0.34% ⬆️0.85%] test-mac-arm
[Failed ⬇️0.36% ⬆️0.0%] ursa-i9-9960x
[Finished ⬇️0.21% ⬆️0.04%] ursa-thinkcentre-m75q
Supported benchmarks:
ec2-t3-xlarge-us-east-2: Supported benchmark langs: Python. Runs only benchmarks with cloud = True
test-mac-arm: Supported benchmark langs: C++, Python, R
ursa-i9-9960x: Supported benchmark langs: Python, R, JavaScript
ursa-thinkcentre-m75q: Supported benchmark langs: C++, Java