Move servoshell code into an internal lib crate

[delan]

This patch moves as much of servoshell’s code as possible from the “servo” bin crate (not to be confused with the “servo” lib crate in libservo) to a “servoshell” lib crate in the same package. As a result:

• we can get rid of ports/servoshell/main2.rs
• you can now distinguish servoshell logging from libservo (RUST_LOG=servoshell vs RUST_LOG=servo)
• cargo timings now list how much time we spend on servoshell’s codegen

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• ./mach build -d does not report any errors
• ./mach test-tidy does not report any errors
• These changes fix #___ (GitHub issue number if applicable)

• There are tests for these changes OR
• These changes do not require tests because there are no functional changes

[mrobinson]

Did you also mean to remove main2.rs in this change as well?

[atbrakhi]

Did you also mean to remove main2.rs in this change as well?

main2.rs is lib.rs now