add `TermExt` for `gridplot`

[codecov-commenter]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (8accdbc) 99.75% compared to head (b80dd94) 99.75%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #359 +/- ## ======================================= Coverage 99.75% 99.75% ======================================= Files 35 36 +1 Lines 2052 2059 +7 ======================================= + Hits 2047 2054 +7 Misses 5 5 ``` | [Impacted Files](https://app.codecov.io/gh/JuliaPlots/UnicodePlots.jl/pull/359?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaPlots) | Coverage Δ | | |---|---|---| | [src/common.jl](https://app.codecov.io/gh/JuliaPlots/UnicodePlots.jl/pull/359?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaPlots#diff-c3JjL2NvbW1vbi5qbA==) | `99.18% <ø> (ø)` | | | [ext/TermExt.jl](https://app.codecov.io/gh/JuliaPlots/UnicodePlots.jl/pull/359?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaPlots#diff-ZXh0L1Rlcm1FeHQuamw=) | `100.00% <100.00%> (ø)` | | | [src/UnicodePlots.jl](https://app.codecov.io/gh/JuliaPlots/UnicodePlots.jl/pull/359?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaPlots#diff-c3JjL1VuaWNvZGVQbG90cy5qbA==) | `100.00% <100.00%> (ø)` | |

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Do you have feedback about the report comment? Let us know in this issue.